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LECTURE NINE

054410 PLANT DESIGN

054410 Plant Design

LECTURE 9: SEPARATION TOWER DESIGN

Daniel R. Lewin Department of Chemical Engineering Technion, Haifa, Israel

Refs: Seider, Seader and Lewin (2004), Chapters 14 and 16 Seader and Henley "Separation Process Principles" (1998), Chaps. 6 and 7 Kister, "Distillation Design" (1992), Chaps. 6 and 7

9-1 PLANT DESIGN - Daniel R. Lewin Separation Tower Design

Lecture Objectives

After this lecture, you should be:

Familiar with the constraints affecting the performance of trayed distillation column. Able to estimate the efficiency of a trayed distillation column Able to compute the optimal diameter of a trayed distillation column. Able to define all of the dimensions of a distillation column, including the minimum wall thickness. For a review of distillation, see: a) Multimedia section on HYSYS-Separations b) http://lorien.ncl.ac.uk/ming/distil/distil0.htm

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Distillation Column Design Overview

Steps involved: Selection of operating pressure, to allow the usage of cooling water for condenser, if possible.

Short-cut method used to estimate RR, number of ideal stages, NT,I = NR,I + NS,I , and location of feed tray. Rigorous solution of material and energy balances to meet the number of specifications = DOFs.

Estimate tray efficiency, E0, and number of actual trays: NR,A = NR,I E0 and NS,A = NS,I E0 Estimate tower height, diameter, and wall thickness. It is assumed that you are familiar with steps , and . This lecture focuses on steps and .

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Focus of this lecture...

The focus of this lecture is on the additional details required to permit the mechanical design of multicomponent separation towers.

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

A Look Inside a Distillation Column

Liquid Outlet weir

a

a a a

Active tray area Downcomer

Vapor

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Separation Tower Design

Bubble-caps, Valves or Sieves...

Bubble-cap tray

Relative cost Pressure drop Efficiency Vapor capacity Typical turndown ratio

9-6

Valve tray

Bubble-caps 2.0 Highest Highest Lowest 5 Valves 1.2 Intermediate Highest Highest 4

Sieve tray

Sieves 1.0 Lowest Lowest Highest 2

Separation Tower Design

PLANT DESIGN - Daniel R. Lewin

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Bubble-caps, Valves or Sieves...

Bubble-cap tray

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Separation Tower Design

Bubble-caps, Valves or Sieves...

Valve tray

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Bubble-caps, Valves or Sieves...

Sieve tray

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Separation Tower Design

Tray Performance Constraints

Adverse vapor/liquid flow conditions can cause: Foaming Entrainment Flooding Weeping/dumping Downcomer flooding

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PLANT DESIGN - Daniel R. Lewin

Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Tray Performance Constraints

Foaming Foaming refers to the expansion of liquid due to passage of vapor or gas, caused by high vapor flow rates.

Although it provides high interfacial liquid-vapor contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whatever the cause, separation efficiency is always reduced.

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Tray Performance Constraints

Entrainment Caused by excessively high vapor flow rates. Entrainment refers to the liquid carried by vapor to the tray above.

It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. Excessive entrainment can lead to flooding.

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Tray Performance Constraints

Flooding

Flooding is brought about by excessive vapor flow, causing liquid to be entrained in the vapor up the column. The increased pressure from excessive vapor also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

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Tray Performance Constraints

Weeping/Dumping Caused by excessively low vapor flow. The pressure exerted by the vapor is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping - the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Tray Performance Constraints

Downcomer Flooding Caused by excessively high liquid flow and/or a mismatch between the liquid flow rate and the downcomer area. This can be avoided by ensuring that the downcomer back-up (level) is below 50% of the tray spacing. This can be checked by performing tray sizing using a process simulator. If necessary, design multipass trays (see later).

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Separation Tower Design

Tray Efficiency Estimation

The actual number of trays required for a particular separation duty is determined by the efficiency of the plate. Any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction.

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Empirical Efficiency Estimation

O'Connell correlation: EO = 0.492 ( µL )

-0.245

± 10%

µ L = viscosity = relative volatility at average column conditions of key component

Separation Tower Design

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Example 1: Tray Efficiency Calculation

Estimate the tray efficiency for the simulated column shown in the table below.

LH = 1.945

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Example 1: Tray Efficiency Calculation

Solution. The average column temperature is (70 + 309)/2 = 190 oF. The closest match to this temperature is at stage 8, at which the viscosity is 0.133 cP (note that the viscosity does not change all that much over the entire column).

Hence, EO = 0.492 ( µL )

-0.245 -0.245

= 0.492 ( 0.133 × 1.945 )

= 0.69 Given that the estimate is subject to ±10% error, a reasonable estimate would be 0.62. Thus, the total number of trays will be:

29 × 7 18 = 11 trays in the rectifying section 18/0.62 = 29 trays 29 × 11 18 = 18 trays in the stripping section

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Tray Section Capacity

Defining column diameter.

Most of the factors that affect column operation are due to vapor flow conditions: either excessive or too low. Vapor flow velocity is dependent on column diameter. Weeping determines the minimum vapor flow required while flooding determines the maximum vapor flow allowed, hence column capacity. If the column diameter is not sized properly, the column will not perform well. Not only will operational problems occur, the desired separation duties may not be achieved.

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Daniel R. Lewin, Technion

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054410 PLANT DESIGN

Estimating Flooding Velocity

The flooding velocity is computed based on a force balance on a suspended liquid droplet. This is the critical velocity at which liquid droplets become suspended, a result of a perfect balance between gravitational, buoyant and drag forces (Sounders and Brown, 1934): drag

L

( )

3 d p

6

g

- G

( )

3 d p

6

g - CD

( )2

2 d p

uf2

4

G

=0

buoyancy

gravity

buoyancy

drag

Solving for flooding velocity:

uf = C

L - G G

12

4d p g where C = 3CD

gravity

uf

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Separation Tower Design

Estimating Flooding Velocity

In practice, C is treated as an empirical parameter determined using experimental data.

C = C SB FST FF FHA

where CSB is an empirical function of the ratio:

FLG = ( L G ) G L

and FST = ( 20 )

0.2

, = liquid surface tension [dyne/cm]

1, for non-foaming systems (e.g., most distillation applications) FF = 0.5-0.75, for foaming systems (e.g., absorption with heavy oils. 1, for Ah Aa 0.1 FHA = 5 ( Ah Aa ) + 0.5, for 0.06 Ah Aa < 0.1

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Estimating Flooding Velocity

uf = C SB ( 20 ) FF FHA

0.2

L - G G

12

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PLANT DESIGN - Daniel R. Lewin

Separation Tower Design

Tray Section Capacity

Tower inside cross-sectional area, AT, is computed at a fraction f (typically 0.75-0.85) of the vapor flooding velocity, uf : G = (fuf ) (A - Ad ) G (14.10) T

Substituting AT = (DT ) 4 into Eq.(14.10) and solving

2

for DT :

4G DT = (fuf ) (1 - Ad AT ) G

1/2

(14.11)

0.1 , (F - 0.1) , Ad = 0.1 + LG AT 9 0.2 ,

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0.1 FLG 1.0 FLG = ( L G ) G L FLG 1.0

Separation Tower Design

FLG 0.1

PLANT DESIGN - Daniel R. Lewin

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054410 PLANT DESIGN

Selection of Multipass Trays

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Separation Tower Design

Example 2: Tray Diameter Calculation

Compute the diameter of a valved-distillation column with the following data - Liquid phase: = 7.1 dyne/cm, L = 215,000 lb/hr, L =32.4 lb/ft3, Vapor phase: G = 244,000 lb/hr, G =1.095 lb/ft3. Solution.

FLG = (215,000/244,000)(1.095/32.4)0.5 = 0.162 From Slide 9-23 , for 24" tray spacing, CSB = 0.09 m/s slide 9-23 FF = 1 (no foaming), FHA = 1 (valves), so:

Uf = 0.09 ( 7.1 20 )

Ad AT = 0.1 + (FLG

32.24 - 1.095 = 0.39 m/s = 4,610 ft/hr 1.095 - 0.1 ) 9 = 0.107. Assuming operation at 80% flooding:

0.2

(1 )(1 )

4 (244, 000 ) DT = = 9.3 ft 0.8 ( 4, 610 ) (1 - 0.107 ) 1.095 Note: (a) In general, diameters in rectifier and stripper may differ. (b) If DT < 2 ft, use a packed column.

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1/2

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054410 PLANT DESIGN

Example 2: Tray Diameter Calculation

For this large diameter column, we should consider installing a multipass tray. Recall from data: L = 215,000 lb/hr and L =32.4 lb/ft3 = 4.33 lb/gal Volumetric flow rate = (215,000/60 ) / 4.33 = 828 gpm

9.3

828

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Separation Tower Design

Example 3: Tray Diameter Calculation

Compute the tray diameter for the simulated column shown in the table on Slide 9-18 . Assume valve trays and light slide 9-18 hydrocarbon service. FF = 1 (no foaming), FHA = 1 (valves) Solution. The first thing we need to do is to identify the critical tray in both the rectifier and stripping sections, defined as the trays in which the loads for each section are maximized. Rectifier Section - based on Stage 3 Stripper Section - based on stage 19

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LECTURE NINE

054410 PLANT DESIGN

Example 3: Tray Diameter Calculation

Rectifier Section (based on Stage 3).

FLG = (85,360/121,184)(2.478/27.944)0.5 = 0.2098

Uf = 0.21 (3.3 20 )

0.2

(1 )(1 )

27.979 - 2.478 = 0.47 m/s = 5,551 ft/hr 2.478

Stripper Section (based on stage 19).

Uf = 0.16 (2.84 20 )

0.2

Ad AT = 0.1 + (FLG - 0.1 ) 9 = 0.112. Assuming operation at 80% flooding: 1/2 4 (121,184 ) DT,R = = 3.97 ft 0.8 ( 5, 551 ) (1 - 0.112 ) 2.478

FLG = (185,434/129,112)(3.614/27.191)0.5 = 0.5236

(1 )(1 )

Ad AT = 0.147. Hence, DT,S

27.191 - 3.614 = 0.277 m/s = 3,272 ft/hr 3.614 1/2 4 (129,112 ) = = 5.40 ft 0.8 (3,272 ) (1 - 0.117 ) 3.614

Since the difference more than 20%, note that the rectifier diameter is 4 ft- 29 the stripper - diameter is 5.5 ft (to nearest ½'). 9 and PLANT DESIGN Daniel R. Lewin Separation Tower Design

Estimating Column Pressure Drop

Typically, tray pressure drop for flow of vapor in a tower is between 0.05-0.15 psi/tray. For a sieve tray, the head loss is due to the friction for vapor flow through the tray perforations, the holduo of the liquid, and the loss due to surface tension: ht = hd + h + h

ht = total pressure drop [in] h = equivalent head on tray [in] hd = dry tray pressure drop [in]

h = pressure drop due to s.t. [in]

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Daniel R. Lewin, Technion

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054410 PLANT DESIGN

Estimating Column Pressure Drop

Dry sieve tray pressure drop is computed using a modified orifice equation:

uo2 G 2 C o L

hd = 0.186

u0 = hole velocity [ft/s]

C 0 - depends on percent hole area and the ratio of tray thickness

to hole diameter. Range: 0.65-0.85. Typical value: 0.73.

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Separation Tower Design

Estimating Column Pressure Drop

Equivalent height of clear liquid holdup on tray: 23 q h = e hw + C L w L e

hw = weir height [in]

e = effective relative froth density (ht. of clear liquid/froth height) = exp ( -4.257KS0.91 ) G L - G = superficial vapor velocity [ft/s] based on active bubbling area, Aa = A - Ad T = weir length [in] (for Ad A = 0.1, taken as 73% of DT ) T = liquid flow rate across tray [gal/min] = 0.362 + 0.317 exp ( -3.5h ) W

PLANT DESIGN - Daniel R. Lewin Separation Tower Design

KS = capacity parameter [ft/s] = ua

Aa LW qL C

12

ua

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Estimating Column Pressure Drop

As the gas emerges from the tray perforations, the bubbles must overcome surface tension. The pressure drop due to the surface tension is given by the difference between the pressure inside the bubble and that due to the liquid: 6 h = g LDB (max )

Generally, the maximum bubble diameter, DB(max), may be taken as the tray hole diameter.

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Separation Tower Design

Example 4: Estimating Tray P

Estimate the tray vapor pressure drop for a 1m diameter absorber equipped with sieve trays. Given: hw = 2", DH = 3/16" Liquid phase: = 70 dyne/cm, L = 2,883 kg/hr, L = 986 kg/m3 Vapor phase: G = 7,920 kg/hr, G = 1.92 kg/m3. Solution. At the bottom of the tower, vapor velocity based on the total cross-sectional area of the tower is:

7, 920 3, 600

(1.92) (1 )

2

4

= 1.46 m/s

For a 10% hole area, based on total cross-section of the tower: 1.46 u0 = = 14.6 m/s = 47.9 ft/s 0.1 47.92 1.92 Hence, hd = 0.186 = 1.56" 2 0.73 986

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Example 4: Estimating Tray P

Solution (cont'd). Taking weir length as 73% of DT gives LW = 0.73 m = 28.7"

Liquid flow rate in gpm, qL = 2, 883 60 = 12.9 gpm 986 × 0.003785

12

Ad/AT = 0.1, Aa/AT = 0.9, so ua = 1.46/0.9 = 1.62 m/s = 5.32 ft/s.

KS = ua ( G

( L - G ) )

= 0.235 ft/s

e = exp ( -4.257KS

0.91

) = 0.32

C = 0.362 + 0.317 exp ( -3.5h ) = 0.362 W

23 Hence, h = e hw + C (qL ( L e ) ) = 0.67 " w

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Separation Tower Design

Example 4: Estimating Tray P

Solution (cont'd). Assuming DB(max) = DH = 3/16" = 0.00476 m = 70 dynes/cm = 0.07 N/m = 0.07 kg/s2 and g = 9.8 m/s2

Hence, h =

g LDB (max )

6

= 0.000913 m = 0.36"

Thus, total head loss/tray, ht = hd + h + h

= 1.56 + 0.67 + 0.36

= 2.59" Recalling that L = 986 kg/m3 = 0.0356 lb/in3

Thus, the tray vapor pressure drop = htL = 0.092 psi

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Complete Column Sizing

Disengagement

4 ft

Rectifying Section

2×Nr ft

Dr ft

Stripping Section

2×Ns ft 10 ft

Ds ft

Sump Skirt

9 - 37

Maximum height of column = 175', Maximum L/D ratio = 30

PLANT DESIGN - Daniel R. Lewin

Separation Tower Design

ASME Pressure Vessel Code

In the absence of wind and earthquake conditions and excluding vacuum operation: 12Pd DI Tp = (16.60) 2 S E - 1.2 Pd Tp = wall thickness [in] to withstand internal pressure Pd = internal design pressure [psig] DI = inside shell diameter [ft] S = maximum allowable stress at design temp. [psig] E = weld integrity (E = 0.85 for wall thicknesses < 1.25".

A value of 1 is used for thicknesses more than 1.25")

Recommended material CS SA-285, grade C 1%Cr, 0.5%Mo Steel, SA-387B 1%Cr, 0.5%Mo Steel, SA-387B 1%Cr, 0.5%Mo Steel, SA-387B Conditions -20-650 oF no H2 -20-750 oF with H2 to 800 oF with H2 to 900

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oF

S (psig) 13,750 15,000 14,750 13,100

with H2

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

ASME Pressure Vessel Code

For vertical vessels, the vessel walls need to withstand wind load, computed using: 0.22 (Do + 18 ) L2 Tw = 2 where Do is outside shell diameter (inches), L is vessel height (tangent to tangent length, in inches), and the factor of 18 allows for the column cage ladders, which adds additional effective diameter to the column. When there is wind load, the girth seam must withstand the combined load of the wind and the internal pressure, the latter computed using: 12Pg DI Tg = 2SE + 0.4Pg An estimate for the required wall thickness at the bottom of the tower is then: Tb = Tw + Tg

9 - 39 PLANT DESIGN - Daniel R. Lewin Separation Tower Design

SDo

ASME Pressure Vessel Code

To estimate the vessel thickness (assumed constant), use the average of the top and bottom thicknesses, plus the corrosion allowance, Tc, usually 0.125". Thus the values of wall thickness are computed as follows: HORIZONTAL VESSELS. VERTICAL VESSELS.

Ts =Tp +Tc Ts = 0.5 (Tb +Tp ) +Tc

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Separation Tower Design

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

ASME Pressure Vessel Code

At low pressures, wall thickness computed using the above equations may be too small to give sufficient rigidity to vessels. The minimum wall thickness below should be used.

DI [ft] Up to 4 4-6 6-8 8-10 10-12 Minimum value for tp [in]

1/4 5/16 3/8 7/16 1/2

Finally, the values computed need to be rounded up to the nearest standard plate thickness, as given by the table below:

Ts up to [in] 1 2 3 >3

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Rounding increment [in]

1/32 1/16 1/8 1/4 Separation Tower Design

Example 5: Wall Thickness Calculation

Compute the wall thickness for a distillation column with height 175 ft and inside diameter 10 ft. The operating pressure is 110 psia and 150 oF at the bottom of the tower and 100 psia and 120 oF at the top. Material of construction is CS. Solution. Design basis: Pd = 1.2×max P = 1.30×(110-14.7) = 123 psig Td = max T + 50 oF = 200 oF. Using Eq. (16.60) assuming CS shell: 120 × 123 Tp = = 0.635" 2 × 13,750 × 0.85 - 1.2 × 123 The vessel thickness at the bottom of the tower is: 0.22 (10 × 12 + 18 ) 1752 123 × 10 × 12 Tb = + = 1.21" 2 13, 750 × 10 2 × 13, 750 × 1.0 + 0.4 × 123

Thus: Ts = 0.5 Tb +Tp +Tc = 0.5 (1.21 + 0.635 ) + 0.125 = 1.049"

(

)

Rounding up, this gives Ts = 1.0625" (1 1/16")

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Daniel R. Lewin, Technion

LECTURE NINE

054410 PLANT DESIGN

Summary

After reviewing the materials of this lecture, you should be: Familiar with the constraints affecting the performance of trayed distillation column. Able to estimate the efficiency of a trayed distillation column. Able to compute the optimal diameter of a trayed distillation column. Able to define all of the dimensions of a distillation column, including the minimum wall thickness.

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Daniel R. Lewin, Technion

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