Design of Absorber

Chapter 5

Equipment Design Absorber

EQUIPMENT DESIGN

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Chapter 5


DESIGN OF ABSORBER

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Chapter 5


DESIGN OF ABSORBER

Absorber The removal of one or more selected components from a mixture of gas by absorption into a suitable solvent.

Selection of Solvent The most commonly used solvents for the removal of hydrogen sulfide & CO 2 from the gaseous streams are amines, especially alkanolamines. A number of different compounds belonging to this category can be used for this purpose. Some of the most commonly used amines are compared in the following lines. Mono-Ethanol Amine: •

Preferred for low concentrations without contaminants COS & CS2.

•

Easy regeneration.

•

Very corrosive in nature.

•

High heat of reaction with H2S.

•

Relatively high vapor pressure. •

Relatively low cost.

Di-Ethanol Amine: •

Used for the purification of natural gas even with contaminants COS & CS2.

•

Low vapor pressure.

•

Very less corrosive than MEA.

•

Gives excellent loading (0.7-1 mol of gas/mol of DEA) at high concentrated aqueous solution from 25-50%.

•

Easy recovery.

•

Relatively low cost.

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Tri Ethanol Amine •

TEA is less reactive with acid gases.it has less acid gas carrying capacity.

•

And also cannot reduce H2S content to general pipeline specifications.

•

But the advantage of TEA is that it is selective for H2S.

Selection Criteria Sufficient data and operating experience with several alkanolamines are on hand to permit a judicious selection of the treating solution for a wide range of conditions. An ideal absorbent should meet the following criteria: •

Have a high degree of solubility for the solute (minimizes absorbent required).

•

Have low volatility (increases solute recovery and reduces absorbent loss)

•

Be stable (reduces need to replace absorbent).

•

Be non-corrosive (reduces need for corrosion resistant equipment).

•

Be non-foaming when in gas contact (reduces size of equipment).

•

Be nontoxic and non-flammable (safety).

Choice between Plate & Packed column Vapor liquid mass transfer operation may be carried either in plate or packed column. These two types of operation are quite different. The relative merits of plate over packed column are as follows: 1-Plate column are designed to handle wide range of liquid flow rates without flooding. 2-If a system contains solid contents; it will be handled in plate column, because solid will accumulate in the voids, coating the packing materials and making it ineffective. 3-Dispersion difficulties are handled in plate column when flow rate of liquid are low as compared to gases. 4-For large column heights, weight of the packed column is more than plate column. 5-If periodic cleaning is required, man holes will be provided for cleaning. In packed columns packing must be removed before cleaning. 6-For non-foaming systems the plate column is preferred.

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7-Design information for plate column is more readily available and more reliable than that for packed column. 8-Inter stage cooling can be provided to remove heat of reaction or solution in plate column. 9-When temperature change is involved, packing may be damaged.

Choice of Plate Type There are three main types, sieve tray, bubble cap, valve tray. I’ve selected sieve tray because: 1-They are lighter in weight and less expensive. It is easier and cheaper to install. 2-Pressure drop is low as compared to bubble cap trays. 3-Peak efficiency is generally high. 4-Maintenance cost is reduced due to the ease of cleaning. [2]

Sieve Trays Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters. Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.

Factors affecting the absorption Column VAPOR FLOW CONDITIONS Adverse vapor flow conditions can cause: 1. Foaming 2. entrainment 3. weeping/dumping 4. flooding

Foaming Foaming refers to the expansion of liquid due to passage of vapor or gas. 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. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but 50

Chapter 5


is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced.

Entrainment Entrainment refers to the liquid carried by vapor up to the tray above and is again caused by high vapor flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.

Weeping / Dumping This phenomenon is caused by 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. That is 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.

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 down comer, 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.

State of trays & packing Remember that the actual number of trays required for a particular separation duty is determined by the efficiency of the plate. Thus, 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.

Column Diameter 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. Thus, if the column diameter is not sized properly, the column will not perform well.

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Material Balance across the Absorption Column

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Standard Design Steps

1. Calculation of theoretical number of stages. 2. Calculation of actual number of stages. 3. Calculation of diameter of the column. 4. Calculation of weeping point. 5. Calculation of pressure drop. 6. Down comer Design. 7. Residence Time and Entrainment Calculation. 8. Calculation of the height of the column.

1. Calculation of theoretical number of stages. Now evaluating no of stages from the Kremser method

[3] N=5 Efficiency of column;

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[4] Efficiency of column=75 %

2. Calculation of actual number of stages. Efficiency the column is; Eo = 75% So total number of stages = 7

3. Calculation of diameter of the column. Flooding velocity is given by

[6] Where, = flooding vapor velocity, m/s, based on the net column cross-sectional area An = a constant obtained from figure 11.27 vol. 6 Coulson & Richardson’s “Chemical Engineering”

Where = liquid mass flow-rate, kg/sec

= vapor mass flow-rate, kg/sec

So,

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FLV = 0.376 So K1 K1 =0.05 Hence Uf = 0.209 m/s Max. Vapor Volumetric Flow rate is given by (Vm*M.W/Pv*3600) Vmax =0.135mm3/sec So net area required is = 0.758 m2 Take down comer area as 12% So the column cross-sectional area is At = 0.862 m2 Hence the column diameter is given by D = 1.05m

Methane ethane Propane I-butane n-butane

mol fraction

molecular wt

Lb/mol

Tc,₀R

0.889 0.0096 0.0022 0.0006 0.0005

16 30 44 58 58

14.23 0.29 0.097 0.0348 0.029

343.3 549.8 666 734.1 765.3

yiTc,₀R 305.193 7 5.27808 1.4652 0.44046 0.38265

Pc,Psia 673.1 708.3 617.4 529.1 550.7

yiPc,Psi a 598.385 9 6.79968 1.35828 0.31746 0.27535 55

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I-pentane n-pentane n-hexane

0.0003 0.0003 0.0015

72 72 86

0.022 0.022 0.129

829.8 845.6 914.1

0.24894 0.25368 1.37115

483 489.5 439.7

0.1449 0.14685 0.65955

H2S

0.0001

34

0.0034

672.4

1306

0.1306

CO2

0.0533

44

2.3452

547.7

0.06724 29.1924 1

1073

57.1909

N2

0.0419

28

1.173

226.9

9.50711

20.6148

Pseudo critical temp

353.400 62

492 Pseudo critical pressur e

915

18.3756

3.51

575

379

3.5Lb/ft3

14.7 0.9

550

From fig. ρ=PM/ZR Z =0.9 T Tr

1.62704 9 56.6Kg/m 3

Pr

915

686.024 3

Now Plate design is given by

Column Diameter = Dc = 0.69 m Column Area = Ac = 0.86 m2 Down Comer Area = Ad = 0.103 m2 Net Area = An = 0.758 m2 Active Area = Aa = 0.655m2 Hole Area = Ah = 0.033 m2 Take, Weir height = 50 mm Hole diameter = 5 mm Plate thickness = 5 mm Total number of holes is given by = Area of holes / area of one hole 56

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So, Total number of holes is =1680

4. Calculation of weeping point. For the calculation of weeping hole diameter must be selected so that at lowest operating rate the vapor flow velocity is still above the weep point.

[7]

Where = minimum vapor velocity through the holes, m/s = hole diameter, mm a constant, dependent on the depth of clear liquid on the plate, obtained from figure 11.30 vol. 6 Coulson & Richardson’s “Chemical Engineering”

Where = weir length, m = weir crest, mm liquid = liquid flow-rate, kg/s

At 70% of maximum liquid rate. So Min how = 36.16 mm liq. K2 from graph 11.30 is

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K2 = 30.9 So Uhmin = 1.6 m/sec Actual minimum vapor velocity is = 2.1 m/sec So the column is operating above weep point

5. Calculation of pressure drop. It is convenient to express the pressure drop in terms of millimeters of liquid. But in pressure units it is given by Where = total pressure drop, Pa (N/m2) = total pressure drop, mm liquid

Dry pressure drop is given by

Where = dry plate pressure drop = orifice coefficient = vapor velocity

So hd = 71.12 mm liq. Residual head is given by

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Where = residual head

hr = 11.87 mm liq. Total pressure drop is given by

Where, = total plate pressure drop = dry plate pressure drop = height of the weir = weir crest, mm liquid

ht = 169.15 mm liq. In pressure units 1747 Pa (N/m2)

6. Down comer Design. The down comer area and the plate spacing must be such that the level of the liquid and froth in the down comer is well below the top of the outlet weir on the plate above. If the level rises above the outlet weir the column will flood.

Where = down comer back-up, measured from plate surface, mm = head loss in the down comer, mm

Where 59

Chapter 5


= head loss in the down comer, mm = liquid flow-rate in the down comer, kg/s = either down comer area Ad or the clearance area under the down comer Aap which is smaller

Where, = clearance area under the down comer = height of the bottom edge of the apron above the plate = length of the weir

40 mm So 23.73 mm And = 0.24 m

As Where = down comer back-up, measured from plate surface, mm = plate spacing

So The tray spacing is acceptable.

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7. Residence Time and Entrainment Calculation. Residence time in the down comer is given by

Where = residence time, s = down comer area = clear liquid back-up, m = liquid flow-rate in the down comer, kg/s

So tr = 5.3 sec As it is well above 3 sec so design is acceptable.

For the check of entrainment evaluate Uv = Uv = 0.18 m/sec So % flooding = 84.90% FLV we know so using graph 11.29 Ψ = 0.004 It is well below 0.1 so there is no chance of entrainment. & the Process is satisfactory.

8. Calculation of the height of the column

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No. of plates = 7 Tray Spacing = 0.5 m Tray Thickness = 0.005 m Total Thickness of Trays = 0.035 m Top Clearance = 1 m Bottom Clearance = 1 m Total Height = 5 m

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Specification Sheet Equipment : Operation: Function:

Absorption Column Continuous Removal of Acid gases

Design Data: No. of trays Efficiency Diameter

7 75% 1.05 m

Height No. of holes Flooding No weeping Fractional entrainment Pressure Design Pressure Tray Thickness Weir Height Weir Length Tray Spacing Hole Diameter Area of one Hole Total Hole Area Down Comer Area Net Area Material of construction Head type

5m 1680 84.9% 0.004 6300 kPa 7560 kPa 0.005 m 0.05 m 0.78 m 0.457 m 0.005 m 0.00002 m2 0.033 m2 0.103 m2 0.758 m2 Stainless steel 316 Ellipsoidal

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Design of Absorber

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