Distillation Column 1-2-3 - Sizing

8/10/2019 Distillation Column 1-2-3 - Sizing

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Distillation Column

Figure 1. P&ID for Distillation

Separation process is done inside the distillation unit. The main goal of this process is

separating the levulinic acid, the main product, from other side products and impurities so the

higher purified percentage of levulinic acid will be obtained. The side products, which are formic

acid and furfural, are also separated in a two different distillation columns. Walas (1988) stated

several rules of thumb for selecting and designing appropiate column control:

Distillation usually is the most economical method of separating liquids, superior to

extraction, adsorption, crystallization, or others.

For ideal mixtures, relative volatility is the ratio of vapor pressures 12= P 2 /P 1. Tower operating pressure is determined most often by the temperature of the

available condensing medium, 100-120F if cooling water; or by the maximum

allowable reboiler temperature, 150psig steam, 366F.

Sequencing of columns for separating multicomponent mixtures: (a) perform the

easiest separation first, that is, the one least demanding of trays and reflux, and leave

the most difficult to the last; (b) when neither relative volatility nor feed

concentration vary widely, remove the components one by one as overhead products;

(c) when the adjacent ordered components in the feed vary widely in relative


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volatility, sequence the splits in the order of decreasing volatility; (d) when the

concentrations in the feed vary widely but the relative volatilities do not, remove the

components in the order of decreasing concentration in the feed.

Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio Rm. The economically optimum number of trays is near twice the minimum value N m .

The minimum number of trays is found with the Fenske-Underwood equation:

Nm = log[

Minimum reflux for binary or pseudobinary mixtures is given by the following when

separation is esentially complete ( x D 1) and D / F is the ratio of overhead product

and feed rates:

when feed is at the bubblepoint,

when feed is at the dewpoint.

A safety factor of 10% of the number of trays calculated by the best means is

advisable.

Reflux pumps are made at least 25% oversize. For reasons of accessibility, tray spacings are made 20-24 in.

Peak efficiency of trays is at values of the vapor factor in the range 1,0-1,2 (ft/sec)

vacuum.

The optimum value of the Kremser-Brown absorption factor A = K(V/L)is in the

range 1.25-2.0.

Pressure drop per tray is of the order of 3 in. of water or 0.1 psi. Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60-

90%; for gas absorption and stripping, 10-20%.

Sieve trays have holes 0.25-0.50 in. dia, hole area being 10% of the active cross

section.

Valve trays have holes 1.5in. diaeach provided with a liftable cap, 12-14 caps/sqft of

active cross section. Valve trays usually are cheaper than sieve trays.

Bubblecap trays are used only when a liquid level must be maintained at low

turndown ratio; they can be designed for lower pressure drop than either sieve or

valve trays.


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Weir heights are 2in., weir lengths about 75% of tray diameter, liquid rate a

maximum of about 8gpm/in. of weir; multipass arrangements are used at high liquid

rates.

Packings of random and structured character are suited especially to towers under 3

ft dia and where low-pressure drop is desirable. With proper initial distribution and

periodic redistribulion, volumetric efficienciescan be made greater than those of tiray

towers. Packed internals are used as replacements for achieving greater throughput or

separation inexisting tower shells.

For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2

in.

The ratio of diameters of tower and packing should be at least 15.

Because of deformability, plastic packing is limited to a 10-15 ft depth unsupported,

metal to 20-25 ft.

Liquid redistributors are needed every 5-10 tower diameters with pall rings but at

least every 20ft. The number of liquid streams should be 3-5/sqft in towers larger

than 3 ft dia (some experts say 9-12/sqft), and more numerous in smaller towers.

Height equivalent to a theoretical plate (HETP) for vapor-liquid conlacting is 1.3-

1.8ft for 1in. pall rings, 2.5-3.0 f:for 2 in. pall rings.

Packed towers should operate near 70% of the flooding rate given by the correlationof Sherwood, Lobo, et al.

Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A

takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is slzed

for a linear velocity of that phase of 0.5 ft/sec. minimum diameter of 16in.

For towers about 3ft dia, add 4ft at the top for vapor disengagement and 6 f t at the

bottom for liquid level and reboiler return.

Limit the tower height to about 175ft max because of wind load and foundationconsiderations. An additional criterion is that L/D be less than 30.

The first distillation column will separate formic acid from the levulinic acid and other by

products. Both furfural and formic acid form an azeotrope with water. In this case formic acid

acts as an entrainer to make the separation feasible. Levulinic acid, formic acid, and water leave

the column at the bottom while formic acid with high recovery percentage leaves at the top. The


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column is operated at 1,01 bar because there is no need for a high pressure and the second

column also operates at 1,01 bar so no compressors or expanders are needed. The second (C02)

and third (C03) column separates furfural and levulinic acid from its mixture. The disadvantage

of these separation methods is the presence of an entrainer in the mixture, which has to be

removed to obtain pure levulinic acid without any impurities, including water.

Formic acid and water form maximum boiling containing 77.5% acid at 101.3 kPa and

83.2 % acid at 2.4 MPa. At the 101.3 kPa, the azeotropic mixture boils at 380.3 K, and at 2.4

MPa it boils at 407.8 K. This dependence upon pressure makes it possible to produce

concentrated formic acid using pressure shift distillation. The feed liquor is pumped to a column

operated at 300 kPa producing nearly pure water as distillate. The bottom product is fed to a

vacuum (20 kPa) column producing nearly pure formic acid as distillate. The bottom product

from the vacuum column is circulated to the pressurized column. The temperatures of all feeds

entering the column are at the bubble point temperature of the feed. This results in an optimal

separation. (Girisuta, 2006)

A. Column Material Selection

It is allowed to use metal material for non-food products, but it is essential to select a

material that is non-corrosive because the distillation process will happen in high temperature,

thus evaporating water content from the mixture. The material chosen is carbon steel (CS)

because of the total pressure drop allowance in column. This criterion is based on ASME B31.4and the rules of thumb that has been explained before.

B. Calculation Methods (Fenske-Underwood-Gilliand Method)

The first step in the design of distillation equipment is specification of the required

distribution of light and heavy key components. Then the specific operating conditions and

equipment size are established, ultimately on the basis of an economic balance or simply by

exercise of judgment derived from experience. The design parameters that need to be determined

include intermediate ones such as limiting reflux and trays that are needed for establishing a

working design. These design parameters are the following:

Minimum number of theoretical trays Distribution of nonkeys between the overhead and bottoms products Minimum reflux


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Operating reflux Number of theoretical trays Location of the feed tray Tray efficiencies.

B.1. Distillation Column I (C-101)

1. Stream Composition

Basis = 1 hour, feed = 13890,254 kg/hour

Table 1. Feed, distillate, and bottoms composition

Component Feed (%) Distillate (%) Bottoms (%)Cellulose 0,02 0,00 0,02Formic Acid

1,0199,99

0,02Furfural 1,14 0,01 1,13Glucose 0,04 0,00 0,04Hemicellulose 0,09 0,00 0,10HMF 0,29 0,00 0,29Levulinic Acid 3,62 0,00 3,65Lignin 1,18 0,00 1,19Sodium Hydroxid 0,00 0,00 0,00Water 90,76 0,00 91,69Xylose 1,86 0,00 1,88

100 100 100

2. Temperatures

Dew point of the distillateTable 2. Dew point of the distillate

Component P m Y XFormic Acid (LK) 101,50 1,00 0,99 1,00Furfural (HK) 2,39 0,02 0,01 0,00

1,00 1,00Distillate dew point is 100,40 oC

Where: ,


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Figure 2. Fenske equation for minimum plates expressed in graph form (Source: Gulf, 2002)

From this figure, we can get log N M= 1,28 ; so that, N M= 19,05 trays 19 trays

5. Defining N opt/NMfrom Figure 4.2, for finding N opt .

Figure 3. Relation between optimum-to-minimum ratio and Fenske separation factor (Source: Gulf, 2002)


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From this figure, we can get N opt/NM = 1,84, so that N opt = 1,84 x 19,05 = 35,052 35.By

assuming the tray efficiency is about 85%, we can calculate N actual as (N opt /Tray Efficiency), so

that we get N actual = 41 trays.

6. log[ = log [ After getting this value, we can define R opt/R Mfrom figure 4.3.

Figure 4. Optimum-minimum reflux ratio relationship to the columns feed, distillate, and bottoms

composition (Source: Gulf, 2002)

Based on this figure, if we have log[ ] = 2,20 and, the value R opt/R M= 1,35.

7. Defining the value of by using Figure 4.4

The calculated value of and = = 0,87, then = 1, 58


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Figure 5. Underwoods vs key ratios in feed (Source: Gulf, 2002)

By using Figure 4.5-4.7, the value of ( i.xiD)/(i-) will be obtained as:

Figure 6. Underwoods vs parameter for in range 1,01 to 1,11 (Source: Gulf, 2002)


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Figure 8. Underwoods vs parameter for heavy key and heavier components(Source: Gulf, 2002)


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R M + 1 = ( i.xiD)/(i-) = -6,21

R M = -6,21 1 = -7,21

9. R opt= (-7,21) x (1.35) = -9,73.

10. Tray Spacing = 0.5 m (Heuristic: Stage spacing range 20 24 inch).

11. Height of Tower

20,71 m

(Heuristic: Maximum column height allowed is 175 ft or 53,025 m, so the result is meeting therequirements.)

12. Column Diameter

[ ]

[ ]

Where = Maximum allowable vapour velocity, m/s

= Plate spacing

= Maximum vapor rate, kg/s

= 2,04 m.13. Plate Design

Column Diameter (D c) = 2,04 mColumn Area (A c) = D c2/4 = 3,27m 2

Downcomer Area (A d) = 0.12 x A c= 0,12 x 3,27m 2= 0,39 m 2

Net Area (A n) = (A c A d) = 2,88 m 2

Active Area (A a) = (A c 2Ad) = [2,66 (2 x 0,32)] m 2= 2,49 m 2

Hole Area (A h) = 0.1 Aa(10% estimated from active area) = 0,1 2,49= 0,25 m 2


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Weir Length is define from Figure 11.31 of Coulson and Richardson Book.

(Ad/Ac) x 100% = 12 %

Lw/Dc = 0,76

Lw = 1,55 m.

Figure 9. Relation between downcomer and weir length (Source: Coulson and Richardson, 2002)

Hole Diameter = 0,005 m

Plate Thickness = 0,005 m

Number of Holes = 2,04/(1.965 x 10 -5) = 103816,79 holes.

14. Plate Pressure Drop

Dry Plate Pressure Drop

Maximum vapor velocity through holes:

Uh(max) = Vm/Ah = 8,20m/s

(Ah/Aa)*100 = (0,25/2,49)*100

= 10,04

From Figure 11.34, 6th E d. Coulson and Richardsons if the (A h/Aa)*100 = 10,04, when plate

thickness to plate diameter is 1, then C o= 0.83


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Weight of Head

= 328,70 lb

Calculation of Axial Stress in Shell

16. Specifications and Operation Condition

Table 5. Specifications and operation condition of distillation column I

1. Type Distillation Column2. Material Carbon Steel3. Temperature 102,78 oC4. P top 14,5 psi

5. P bottom 16,78 psi5. Light Key Formic Acid6. Heavy Key Furfural7. Space between Tray 0,5 m8. Column Height 20,71 m9. Column Diameter 2,04 m10. Column Thickness 0,18 in11. Plate Thickness 0,005 m12. Plate Pressure Drop 0,12 psi13. Hole Valve Tray Size 0,005 m14. Number Hole Valve Tray 103816,7915. Hole Area 0,25 m16. Weir Length 1,55 m17. Column Area 3,27 m18. Net Area 2,88 m19. Active Area 2,49 m20. Price $ 65000


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B.2. Distillation Column II (C-102)




Component Feed (%) Distillate (%) Bottoms (%)Cellulose 0,02 0,00 0,02Formic Acid 0,00 0,00 0,00Furfural 1,15 99,97 0,00Glucose 0,04 0,00 0,04Hemicellulose 0,10 0,00 0,10HMF 0,29 0,00 0,29Levulinic Acid 3,65 0,03 3,70Lignin 1,19 0,00 1,20Sodium Hydroxid 0,00 0,00 0,00Water 91,69 0,00 92,75Xylose 1,88 0,00 1,90

100 100 100

2. Temperatures


Component P m Y XFurfural (LK) 150,02 1,00 0,99 1,00Levulinic Acid (HK) 73,00 4,93 0,01 0,00

1,00 1,00Distillate dew point is 103,45 oC

Where: ,

Bubble point of the bottomTable 8. Bubble point of the bottom

Component P (kPa) m Y XCellulose 2,48 0,02 0,00 0,00Formic Acid 2,43 0,02 0,00 0,00Furfural 0,06 0,00 0,00 0,00Glucose 0,00 0,00 0,00 0,00Hemicellulose 2,48 0,02 0,00 0,00HMF 0,19 0,00 0,00 0,00


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Levulinic Acid 0,19 0,00 0,00 0,04Lignin 2,48 0,02 0,00 0,01Sodium Hydroxid 0,00 0,00 0,00 0,00Water 110,41 1,09 1,01 0,93Xylose 2,48 0,02 0,00 0,02

1,01 1,00Bottom bubble point is 170 oC.

where, , so that 136,73 oC.3. Relative Volatilities

Relative Volatility of each component is defined base to T av = 136,73 oCand HK as the

base .

Table 9. Relative volatilities of the distillate

Component P m Furfural (LK) 1,00 1,00 1,80Levulinic Acid (HK) 0,56 0,02 3,62Where,

4. log[ = log [

From Figure 4.1, we can define N M.


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Figure 10. Fenske equation for minimum plates expressed in graph form (Source: Gulf, 2002)




From this figure, we can get N opt/NM= 1,81, so that N opt = 1,81 x 15,38 = 27,84 28.By assuming

the tray efficiency is about 85%, we can calculate N actual as (N opt/Tray Efficiency), so that we get

Nactual = 33 trays.

6. log[ = log [( ) ( ) ( ) After getting this value, we can define R opt/R Mfrom figure 4.3.


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Figure 12. Optimum- minimum reflux ratio relationship to the columns feed, distillate, and bottoms



7. Defining the value of by using Figure 12

The calculated value of and = = 0,31, then = 1, 94



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Figure 16. Underwoods vs parameter for heavy key and heavier components (Source: Gulf, 2002)

R M + 1 = ( i.xiD)/(i-) = 7,59

R M = 7,59 1 = 6,59

9. R opt= (6,59) x (1,47) = 9,69.


11. Height of Tower

16,66 m(Heuristic: Maximum column height allowed is 175 ft or 53,025 m, so the result is meeting the

requirements.)

12. Column Diameter


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Uh(max) = Vm/Ah

= 7,59m/s

(Ah/Aa)*100 = (0,27/2,69)*100

= 10,04

From Figure 11.34, 6th E d. Coulson and Richardsons if the (A h/Aa)*100 = 10,04, when platethickness to plate diameter is 1, then C o= 0.83

hd = 51 (U h/Co)2 (dv/dl)

= 51 (7,59/0.83) 2(1,00/971,05)

= 4,40 mm liquid

Residual Drop


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hr = 12.5*1000/d l

= 12.5*1000/971,05

=12,87 mm liquid

hw + h ow = 50 + 4,40

= 54,40 mm liquid

Total Plate Pressure Drop

ht = hd + h r + (h w + h ow)

= 4,40 + 12,87 + 54,40

= 71,67 mm liquid

Pt = 9,81*10 -3*ht*dl

= 682,73 Pa

=0,68 kPa = 0,10 psiPtop = 14,5 psi

P bottom = 14,5+ (19*0,10) = 16,50 psi

15. Shell Calculations

Minimum Shell Thickness

inTherefore, 3/16 in thickness can be used

Selection of Head and Head Thickness Calculation

Torispherical Head

Diameter

OD + (OD/24) + 2 sf + 2/3 icr

= 41,73 + 1,74 + (2*3) + (2/3*4)

= 52,14 in

Weight of Head

= 584,11 lb



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16. Specifications and Operation Condition

Table 10. Specifications and operation condition of distillation column II

1. Type Distillation Column

2. Material Carbon Steel3. Temperature 170 oC4. P top 14,5 psi5. P bottom 16,50 psi5. Light Key Furfural6. Heavy Key Levulinic Acid7. Space between Tray 0,5 m8. Column Height 16,66 m9. Column Diameter 2,12 m10. Column Thickness 0,18 in11. Plate Thickness 0,005 m12. Plate Pressure Drop 0,10 psi13. Hole Valve Tray Size 0,005 m14. Number Hole Valve Tray 107888,0015. Hole Area 0,27 m16. Weir Length 1,61 m17. Column Area 3,53 m18. Net Area 3,11 m19. Active Area 2,69 m20. Price $ 65000


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B.3. Distillation Column III (C-103)




Component Feed (%) Distillate (%) Bottoms (%)Cellulose 0,02 0,00 0,02Formic Acid 0,00 0,00 0,00Furfural 0,00 0,00 0,00Glucose 0,04 0,00 0,04Hemicellulose 0,10 0,00 0,10HMF 0,29 0,00 0,30Levulinic Acid 3,70 99,75 0,00Lignin 1,20 0,00 1,25Sodium Hydroxid 0,00 0,00 0,00

Water 92,75 0,25 96,31Xylose 1,90 0,00 1,98

100 100 100

2. Temperatures


Component P m Y XLevulinic Acid (LK) 1,52 0,00 0,01 0,00

Water (HK) 2308,59 1,00 0,99 1,00 1,00 1,00

Distillate dew point is 104,39 oC

Where: ,

Bubble point of the bottomTable 13. Bubble point of the bottom

Component P (kPa) m Y X

Cellulose 2.42 0,02 0,00 0,00Formic Acid 2.39 0,02 0,00 0,00Furfural 0.06 0,00 0,00 0,00Glucose 0.00 0,00 0,01 0,00Hemicellulose 2.42 0,02 0,00 0,00HMF 0.18 0,00 0,00 0,00Levulinic Acid 0.18 0,00 0,00 0,00


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From this figure, we can get N opt/NM= 1,68, so that N opt = 1,68 x 20,42 = 34,31 34.By assumingthe tray efficiency is about 85%, we can calculate N actual as (N opt/Tray Efficiency), so that we get

Nactual = 40 trays.

6. log[ = log [( ) ( ) ( ) After getting this value, we can define R opt/R Mfrom figure 4.3.


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Figure 20. Optimum- minimum reflux ratio relationship to the columns feed, distillate, and bottoms



7. Defining the value of by using Figure 4.4

The calculated value of and = = 0,04, then = 1,79



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Figure 24. Underwoods vs parameter for heavy key and heavier components (Source: Gulf, 2002)

R M + 1 = ( i.xiD)/(i-) = 0,49

R M = 0,49 1 = -0,51

9. R opt= (-0,51) x (1,25) = -0,6375


11. Height of Tower

20,20 m

(Heuristic: Maximum column height allowed is 175 ft or 53,025 m, so the result is meeting the

requirements.)

12. Column Diameter

[ ]

[ ]


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Where = Maximum allowable vapour velocity, m/s

= Plate spacing

= Maximum vapor rate, kg/s

= 2,05 m.13. Plate Design

Column Diameter (D c) = 2,05 m

Column Area (A c) = D c2/4 = 3,31m 2

Downcomer Area (A d) = 0.12 x A c= 0,12 x 3,53 m 2= 0,40 m 2

Net Area (A n) = (A c A d) = 2,91 m 2

Active Area (A a) = (A c 2Ad) = [2,66 (2 x 0,40)] m2

= 1,86 m2

Hole Area (A h) = 0.1 Aa (10% estimated from active area) = 0,1 1,86= 0,19 m 2

Weir Length is define from Figure 11.31 of Coulson and Richardson Book.

(Ad/Ac) x 100% = 12 %

Lw/Dc = 0,76

Lw = 1,56 m.



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Uh(max) = Vm/Ah

= 10,79m/s

(Ah/Aa)*100 = (0,19/1,86)*100

= 10,22

From Figure 11.34, 6th E d. Coulson and Richardsons if the (A h/Aa)*100 = 10,22, when platethickness to plate diameter is 1, then C o= 0.82

hd = 51 (U h/Co)2 (dv/dl)

= 51 (10,79/0.82) 2(1,00/1087,48)

= 8,12 mm liquid

Residual Drop

hr = 12.5*1000/d l

= 12.5*1000/1087,48

=11,49 mm liquid

hw + h ow = 50 + 8,12

= 58,12 mm liquid

Total Plate Pressure Drop

ht = hd + h r + (h w + h ow)

= 8,12 + 11,49 + 58,12

= 77,73 mm liquid

Pt = 9,81*10 -3*ht*dl

= 829,24 Pa

= 0,83 kPa = 0,12 psi

Ptop = 14,5 psi

P bottom = 14,5+ (19*0,12) = 16,78 psi


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15. Shell Calculations

Minimum Shell Thickness

in

Therefore, 3/16 in thickness can be usedSelection of Head and Head Thickness Calculation

Torispherical Head

Diameter

OD + (OD/24) + 2 sf + 2/3 icr

= 40,35 + 1,68 + (2*3) + (2/3*4)

= 50,70 in

Weight of Head

= 611,75 lb


16. Specification and Operation ConditionTable 10. Specifications and operation condition of distillation column III

1. Type Distillation Column2. Material Carbon Steel3. Temperature 220 oC4. P top 14,5 psi5. P bottom 16,78 psi5. Light Key Levulinic Acid6. Heavy Key Water7. Space between Tray 0,5 m8. Column Height 20,20 m9. Column Diameter 2,05 m10. Column Thickness 0,18 in11. Plate Thickness 0,005 m12. Plate Pressure Drop 0,12 psi13. Hole Valve Tray Size 0,005 m14. Number Hole Valve Tray 104326,00


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15. Hole Area 0,19 m16. Weir Length 1,56 m17. Column Area 3,31 m18. Net Area 2,91 m19. Active Area 1,86 m

20. Price $ 65000

Main Variables in Designing Distillation Column

1. Temperature

Temperature is the variable that is prone to change in the distillation column. Reaction

process in the distillation column temperature must be guarded in order to achieve maximum

process. To keep the temperature in the distillation column then used steam. Temperature sensor

is a thermocouple. Controlled variable is the temperature in the distillation column. Control

parameter is steam flow rate. Temperature is controlled at inlet temperature, bottom temperature,

and distillate temperature.

The literature of optimum sequencing of columns is referenced by King (1980, pp. 711-

720) and Henley and Seader (1981, pp. 527-555). For preliminary selection of near optimal

sequences, several rules can be stated as guides, although some conflicts may arise betweenrecommendations based on the individual rules. Any recommended cases then may need

economic evaluations.

Perform the easiest separation first, that is, the one least demanding of trays and

reflux, and leave the most difficult to the last.

When neither relative volatility nor concentration in the feed varies widely, remove

the components one-by-one as overhead products.

When the adjacent ordered components in the process feed vary widely in relative

volatility, sequence the splits in the order of decreasing relative volatility.

When the concentrations in the feed vary widely but the relative volatilities do not,

sequence the splits to remove components in the order of decreasing concentration in

the feed.


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2. Pressure

Pressure is one of the most important variables in the distillation column. Pressure in the

distillation column is meant to be kept as same as or slightly above atmospheric pressure.Pressure changes can occur due to input a continuous distillation column and the reaction in the

distillation column. Excessive pressure can affect the quality of the product and can also be

dangerous if the distillation column exploded due to excess pressure. This to prevent excess

pressure distillation column equipped with a relief valve to release the pressure on the

environment. Controlled variable is the pressure in distillation column. When the pressure

exceeds the set point, then the relief valve on the reactor will open thereby releasing the pressure

of the distillation column. Pressure is controlled at top pressure and bottom pressure.

3. Flow Rate

Flow rate in is one of the most important variables to be controlled in a distillation

column. Flow rate in into the distillation column can affect the composition in the distillation

column which will also affect the yield. Besides this flow rate can also affect the height of the

liquid in the distillation column. Sensors are used to measure the flow rate is orificemeter. Then

the flow rate in is controlled by the controller based on set point. Control the flow rate by the

flow control valve (FCV).

Documents

Distillation Column 1-2-3 - Sizing