Bsorber Esign: 9.1 Objective

CHAPTER NO. 9

Absorber Design

ABSORBER DESIGN 9.1 Objective: The objective of using absorption column here in our process is to absorb maleic anhydride from reaction residues and to produce 40% maleic acid solution in scrubbed liquid (liquid leaving the absorber).

9.2 Absorption: Unit operation where the solute of a gas are removed by being placed in contact with a nonvolatile liquid solvent that removes the components from the gas. In gas absorption a soluble vapors are more or less absorbed in the solvent from its mixture with inert gas. The 'purpose of such gas scrubbing operations may be any of the following; 

For Separation of component having the economic value.



As a stage in the preparation of some compound.



For removing of undesired component (pollution).

Solvent: Liquid applied to remove the solute from a gas stream.

Solute: Components to be removed from an entering stream.

9.3 Absorber System Configuration: Gas and liquid flow through an absorber may be countercurrent, crosscurrent, or cocurrent. The most commonly installed designs are countercurrent, in which the waste gas stream enters at the bottom of the absorber column and exits at the top. Conversely, the solvent stream enters at the top and exits at the bottom. Moreover, countercurrent designs usually require lower liquid to gas ratios than co-current and are more suitable when the loading is higher. In a crosscurrent tower, the waste gas flows horizontally across the column while the solvent flows vertically down the column. As a rule, crosscurrent designs have lower pressure drops and require lower liquid-to-gas ratios than both co-current and 146

CHAPTER NO. 9

Absorber Design

countercurrent designs. They are applicable when gases are highly soluble, since they offer less contact time for absorption. In co-current towers, both the gas and solvent enter the column at the top of the tower and exit at the bottom. Co-current designs have lower pressure drops, are not subject to flooding limitations and are more efficient for fine (i.e., submicron) mist removal. Cocurrent designs are only efficient where large absorption driving forces are available. Removal efficiency is limited since the gas-liquid system approaches equilibrium at the bottom of the tower.

9.4 Types of Absorption:  

Physical Absorption Chemical Absorption

9.4.1 Physical absorption: In physical absorption mass transfer take place purely by diffusion and physical absorption is governed by the physical equilibrium.

9.4.2 Chemical Absorption: In this type of absorption as soon as a particular component comes in contact with the absorbing liquid a chemical reaction take place. Then by reducing the concentration of component in the liquid phase, which enhances the rate of diffusion. The case under consideration is a chemisorption where maleic anhydride is reacted with that of water to produce maleic acid solution. Reaction:

C4H2O3 + H2O

C4H4O4

Heat of reaction = 3.6 x 104 Kj/kgmol

9.5 Types of Absorption Equipment    

Packed Column Tray column Venturi Scrubber Spray Column 147

CHAPTER NO. 9

Absorber Design

9.5.1 COMPARISON BETWEEN PACKED AND PLATE COLUMN    

 

Plate columns can be designed to handle a wider range of liquid and gas flowrates than packed columns. Packed columns are not suitable for very low liquid rates. The efficiency of a plate can be predicted with more certainty than the equivalent term for packing (HETP or HTU). Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns. It is easier to make provision for cooling in a plate column; coils can be installed on the plates. It is easier to make provision for the withdrawal of side-streams from plate columns.



    

If the liquid causes fouling, or contains solids, it is easier to make provision for cleaning in a plate column; manways can be installed on the plates. With small diameter columns it may be cheaper to use packing and replace the packing when it becomes fouled. For corrosive liquids a packed column will usually be cheaper than the equivalent plate column. The liquid hold-up is appreciably lower in a packed column than a plate column. This can be important when the inventory of toxic or flammable liquids needs to be kept as small as possible for safety reasons. Packed columns are more suitable for handling foaming systems. The pressure drop per equilibrium stage (HETP) can be lower for packing than plates; and packing should be considered for vacuum columns. Packing should always be considered for small diameter columns, say less than 0.6 m, where plates would be difficult to install, and expensive.

From the above consideration plate column is selected as the absorber, because in our case the diameter of the column is approximately 2.97 meter which is greater than 3 ft.

148

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Absorber Design

9.6 SELECTION OF PLATE TYPE:

Type

Sieve tray

Valve tray

Bubble cap tray

Capacity Efficiency Turn down

High High About 2:1 , not

High to very high High About 4-5:1

Moderate to high Moderate to high Excellent,better than

Entrainment

suitable to operate

valve trays good at

under variable load

extremly low liquid

Moderate

Moderate

rate High,about three times higher than

Pressure drop Cost

Maintanance Fouling tendency

Moderate Low

Low Low

Moderate About 20% higher

that of sieve trays High High, about 2-3

than that of sieve

times higher than

trays Low to moderate Low to moderate

that of sieve trays Relatevly high High , tends to

Effect of

Low

Low to moderate

collect solids. High

corrosion Availability of

Well known

Rarely available

Well known

design information

From above informtion

I have selected sieve trays for desired degree of

absorption.

9.7 Process Operation

149

CHAPTER NO. 9



   

Absorber Design

Changing the conditions of the absorption column can influence the effectiveness and efficiency of absorption. Some important controllable conditions are as follows: Pressure of the column. Temperature of entering liquid and gas streams. Humidity of the gas stream. Ratio of the liquid and gas stream rates.

Raising the total pressure of the column may increase the efficiency of the separation because increasing the pressure decreases the liquid flow rate and increases the concentration of the gas. The temperature of entering liquid effect absorption in that it effects the flow rate of liquid required for the separation with a given number of stages.Increasing the temperature of the entering solvent increases the liquid flow rate required. Inlet gases of the absorber with high humidity at a high temperature effect the capability of the gas to consume latent heat hindering the absorption process. Therefore, dehumidification of the inlet gas should be considered for absorbers with large heat effects. The ratio of the liquid to gas stream rates in that if the ratio is too low, the solute builds in the upper portion of the column causing a higher temperature profile in the top of the column. As a result, internal cooling maybe necessary for lower liquid to gas ratios.

9.7.1 Desired degree of recovery The amount of solute recovery is generally set by the designer. It may be a recovery to ensure product purity requirements or to satisfy a purity requirement if the recovered solute is a feed stream to another unit.

9.8 Selection of Solvent The ideal absorbent should:  have a high solubility for the solute(s)  have a low volatility to reduce loss of absorbent  have a low viscosity to provide a low pressure drop  be nontoxic  Easily available and not expensive. We have selected 40% maleic acid solution as a solvent to absorb maleic anhydride from mixture of gases.

150

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Absorber Design

9.9 Determination of operating pressure and temperature Operating pressure should be high and temperature low for an absorber, to minimize stage requirements and/or absorbent flow rate and to lower the equipment volume required accommodating the gas flow. But in case of stripper operating pressure should be low and temperature high for a stripper to minimize stage requirements or stripping agent flow rate.

9.10 Heat Effects and need for cooling One of the most important considerations involved in designing a gas absorption tower is to determine whether or not temperatures will vary along the length of the tower, since the solubility of the solute gas depends strongly upon the temperature. Heat effects that may cause temperature to vary from point to point in a gas absorber are:    

the heat of solution the heat of vaporization or condensation of the solvent the change of the sensible heat between the gas and the liquid phases the loss of sensible heat form the fluids to internal and external cooling coils or to the atmosphere via the tower walls

9.11 DESIGN CALCULATIONS  DESIGN STEPS: 151

CHAPTER NO. 9

     

Absorber Design

Find out minimum flow rate of liquid Number of theoretical stages Efficiency of the column Number of actual stages required for seperation Height of the column Cross sectional area and diameter of column

 TRAY DESIGN:             

Entrainment Active area of the plate Hole area Hole size/diameter Plate thickness Weir height Weir length Hole pitch Number of holes Check weeping Plate pressure drop Downcomer liquid backup Residence time in downcomer

 MINIMUM LIQUID FLOW RATE: It is the flow rate of liquid at which seperation can not occure. It is found by mLm y 2 k− y 1 k = mv y 2k −x 1 k Kk and y 1 k=(1−є ) y 2 k where mLm = minimum liquid flow rate mv’’ = vapor flow rate = 2380kgmol/hr Kk= equilibrium value of key compnent (k value) = 0.93 y1k= vapor fraction of key component in top of column = 1*10-4 152

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Absorber Design

y2k= vapor fraction of key component at bottom of column = 6*10-3 x1k= fraction of key component in yhe top liquid = 0 є = solute fraction absorbed = 0.983 Substituting values in above Minimum liquid flow rate = mLm = 2175.7 kgmol/hr Actual liquid flow rate = mL = 1.5 * mLm = 3263.5 kgmol /hr

 NUMBER OF THEORETICAL STAGES: Absorption factor Ai=

L KV =1.47 ≈ 1.5

Using Kresmer Equation For Calculation Of No.Of Stages ( Ai¿¿ N +1− Ai)/( Ai ¿¿ N +1−1)=solute fraction absorbed ¿ ¿ simplifying above equation N +1 log ( Ai )=log

( Ai−∈) (1−∈)

Where Ai = absorption factor Є = solute fraction absorbed Putting values N = 7.70 ≈ 8 stages

 EFFICIENCY OF THE COLUMN: Using Graph (fig 9.1) For The Efficiency Of The Column For Gas Absorption Efficiency of the column = E⁰ = 51%

 NO. OF ACTUAL TRAYS: 153

CHAPTER NO. 9

Na=

Absorber Design

N E° Actual trays =15.71 ≈ 16 tarys

 HEIGHT OF THE COLUMN: Height of column is determined by Hc=(Na−1)Hs+ ∆ H Where Hs = tray spacing = 0.45m ∆H = disengagement region = 3.056m Na = no. of actual trays = 21 trays So Hc = 9.18 m

 CROSS SECTIONAL AREA OF COLUMN: Liquid – vapor flow factor Flv=

Lw √ ρv / ρL Vw

where Lw = liquid mass flow-rate= 24.31 kg/s, Vw = vapour mass flow-rate =19.17 kg/s. рv = density of vapors =1.744kg/m3 рL = density of liquid = 846 kg/m3 subsituting values FLV = 0.05 From graph (fig 9.2) using tray spacing of 0.45 m K1 = 0.08 As this graph is limited to liquid of surface tension less than or equal to 0.02 N/m Surface tension of liquid = 0.05 N/m 154

CHAPTER NO. 9

Absorber Design

Corrected K1 = 0.08{0.05/0.02}0.2 = 0.09 Flooding Velocity Uf =K 1 √( ρL−ρV )/ ρV

subsituting values Uf = 2.11 m/s Actual flooding velocity is 80- 85% of this so Un = 0.8*2.11 = 1.69 m/s

Net column area An=

mv Un

where mv = volumetric flow rate of vapor in m3/sec = 10.5 m3/sec Un = actual flooding velocity = 1.69 m/sec Putting values in above equation An = 6.20 m2 Assuming downcomer occupies 12% of cross sectional area so Ac = An + Ad Ac = An + 0.12Ac Ac = An/0.88 Cross Sectional Area Of Column=Ac = 6.20/0.88 = 7.05 m2

 DIAMETER OF COLUMN: 155

CHAPTER NO. 9

Absorber Design

Dc=

4 Ac π

Subsituting values Dc = 2.99 m ≈ 3.0 m

TRAY DESIGN  LIQUID FLOW ARRANGEMENT: Liquid flow rate in m3/sec = 4*10-2 From graph (fig 9.3) liquid flow arrangement is selected as double pass.

 ENTRAINMENT: Using graph (fig 9.4) % flooding = Un/Uf = 80% Then Ψ

= 0.06 (acceptable below 0.1)

 ACTIVE AREA OF PLATE: Aa = Ac-2Ad Putting values we get Active area of plate = Aa = 6.20 m2

 HOLES AREA: It is rule of thumb that hole area is 10% of that of active area of column. So Ah = 6.20 * .10 156

CHAPTER NO. 9

Absorber Design

= 0.620 m2

 HOLE SIZE OR DIAMETER: typically 5 mm hole size is used so Dh = 5mm

 PLATE THICKNESS: Plate thickness for carbon steel material is 5 mm

 WEIR HEIGHT: Weir height = hw = 50 mm

 WEIR LENGTH: Weir height normally used is 77% of column diameter. So Weir length = 0.77* 3.0 = 2.30

 PERFORATED AREA: LW/DC = 0.8 Then From Graph (fig 9.5 ) ѲC = 108 Angle subtended by plate edge strip = 180 – 108 = 72 mean length of un perforated strip

= (3.68 * 0.05 ) π *72/180 = 3.75 m Area of unperforated edge strip = 0.05 * 3.75 =0.01 m2 Mean length of calming zone =(3.68 – 0.05)sin (54) = 2.37 m Area of calming zone = 2* 2.24* 0.05 = 0.23 m2 Total area of perforations =

157

CHAPTER NO. 9

Absorber Design

(active area-area of unperforated strip –area of calming zone) =5.96 m2 Ah/Ap = 0.1 Then from graph (fig 9.6) Lp/Dh = 2.9 (acceptable b/w 2.5 & 4.0)

 NO. OF HOLES: Area of one hole = π/4 (Dh2) = 1.96 * 10-5 m2 No. of holes

=

area of holes / area of one hole

=

1509 holes

 CHECK WEEPING: maximun liquid rate = 24.31 kg/sec maximum liquid rate at 70% turn down ratio = 0.7 * 24.31 = 17.017 kg/sec

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

Maximum how = 750 [ 24.31/(846∗2.30) ] = 39.706 mm of liquid Minimum how = 750 [ 17.01/(846∗2.30) ] =31.266 mm of liquid At minimum rate = hw+how = 81.266 mm liquid 158

CHAPTER NO. 9

Absorber Design

Then from graph (fig 9.7) K2 = 30.7 minmim design vapor velocity= Uh = 9.34 m/s actual minimum vapor velocity = 11.8394 m/s (minimum operating velocity is well above the weep point)

 PLATE PRESSURE DROP:

Where ∆Pt = total plate pressure drop, Pa(N/m2), ht = total plate pressure drop, mm liquid

 DRY PLATE DROP:

C0 = 0.84 from fig 9.8 at (Ah/Ap)*100 = 11 & unit plate thickness over hole diameter) Minimum vapor velocity through holes Uh = volumetric flow rate of vapor/ hole area = 16.94 m/s then hd = 42.62

 RESIDUAL HEAD: 159

CHAPTER NO. 9

Absorber Design

hr = 14.77541 mm of liquid

 TOTAL DROP: The total plate drop is given by

=147.10 mm liquid

PLATE PRESSURE DROP ∆Pt = 1220.1 N./m2

 DOWNCOMER DESIGN [BACK-UP]: Hap = hw -10 = 40 mm Area under apron = Aap= Hap * weir length Aap = 0.092m2 This is less than Ad so use it as in below equation

where Lwd = liquid flow rate in downcomer = 24.31kg/s, Am = Aap =clearance area under the downcomer= 0.092m2 Putting values hdc = 16.08mm then backup in downcomer will be 160

CHAPTER NO. 9

Absorber Design

hb = 252.88 mm = 0.26m [this is less than 0.5(plate spacing + weir height) so plate spacing is acceptable]

 DOWNCOMER RESIDENCE TIME: The down comer residence time is given by

Where tr= residence time (sec) hbc = clear liquid back-up = 0.26 m so residence time in down comer = 7.65 seconds

9.12 COST ESTIMATION From figure 15-11 and 15-13, given in “Plant Design and Economics for Chemical Engineers” 5th edition, by Max S. Peters, Klaus D Timmerhaus, following are the cost estimated for absorption column. Purchased cost of vertical column without trays = $ 33000 Pressure adjustment factor = 1.6 Material adjustment factor = 1.0 So; Cost of column without trays = $ 33000*1.6*1 = $ 52800 (See fig 9.9 in appendix) Purchased cost/ tray = $ 420 161

CHAPTER NO. 9

Absorber Design

Quantity factor = 1.15 No. of trays = 16 So; Purchased cost of trays = $ 420*17*1.15 = $ 8211 (See figure 9.10 in appendix) Total purchased cost of distillation column = cost of column + cost of trays = $ 52800 + $ 8211 = $ 61011

9.13 Specification sheet 162

CHAPTER NO. 9

Absorber Design

Item: Item number: Type: Operation:

gas absorber T-100 sieve tray continous

Function: absorption of maleicanhydride by 40% maleic acid solution. Column sppecifications

Tray specifications

Liquid flow rate

24.31 kg/s

Active area

6.20m2

Vapor flow rate

19.17 kg/s

Hole area

.620m2

Efficiency

51%

Hole size

.005m

Plate thickness

.005m

2.98m

Weir height

.05m

1506

Weir length

2.30m

No. Of stages Diameter

actual 16

No. Of holes

Pressure of 134KPa column Height of column 9.75 m

Calming zone 0.24m2 area Perforated area 5.94m2

Plate spacing

Pressure drop per 1.2 KPa plate Residence time 7 sec

0.45m

REFERENCES 1. Coulson & Richardson, “Chemical Engineering “ Vol 6, R.K.Sinnott. 163

CHAPTER NO. 9

Absorber Design

2. Max S.Peter Hous D. Timmerhous Ronald. West, “Plant Design & Economics for Chemical Engineers”, Ed # 5th. 3. Stanley M.Walas, “Chemical Process Equipment, Selection & Design”, ButterworthHeiaemamm Series in Chemical Engineer. 4.Carl Branan, “Rule of thumb for Chemical Engineer”, Edition 3rd. 5. Warren L. McCabe, Julian C. Smith, Peter Harriott, “ Unit Operation of Chemical Engineering”, Edition 5th, McGraw-Hill, Inc. 6- Harry Silla, Chemical Process Engineering Design & Economics, Marcel Dekker,Inc. 2003, United States of America. 7- Ernest Ludwig, Applied Process Design For Chemical And Petrochemical Plants (3rd Ed),Vol 2.

164