DETERMINATION OF FLOODING MASS VELOCITY OF AN ABSORPTION COLUMN USING EXCEL

SPREADSHEET AND MATLAB

Ogheneovo EmemeruraiProduction Dept., Warri Refining & Petrochemicals

Company,P.O. Box 4421, Warri, Nigeria.e-mail:ojemems@yahoo.com

Phone: 08038989491

INTRODUCTION

• Gas absorption is a mass transfer operation in which a gaseous component (absorbate) from a gas phase is transferred to a liquid (absorbent) phase through a gas-liquid interface.

• Gas absorbers could be designed either as tray or packed columns.

• Tray types: Sieve , bubble-cap , & valve with weirs and downcomers.

• Figure 1.0 shows a tray-type absorber column.

INTRODUCTION-Tray absorbers

Figure 1.0: Diagram of an absorber and stripper

INTRODUCTION-Packed absorbers

INTRODUCTION-Packed absorbers

• In packed absorbers liquid flows over the surface of the packing while gas flows through the interstices in the packing.

• Packing types: random(e.g. Raschig rings and structured (e.g. MellapakTM ).

• Packed towers almost always have lower pressure drop than comparable tray towers.

• Ratio of tower diameter to packing diameter should usually be at least 15.

INTRODUCTION: Packed absorbers

• Design pressure drops should be as follows(in water/ft packing)

-Absorbers and Regenerators non-foaming Systems 0.25 - 0.40 -Moderate Foaming Systems 0.15 - 0.25 -Fume Scrubbers Water Absorbent 0.40 - 0.60-Chemical Absorbent 0.25 - 0.40- Atmospheric or Pressure Distillation 0.40 - 0.80- Vacuum Distillation 0.15 - 0.40 - Maximum for Any System 1.0

Theory of operation

• The gas and liquid streams may be arranged in cocurrent or countercurrent flow. The former is common when the absorbed gas reacts chemically in the liquid phase, but in general countercurrent flow is used.

• Countercurrent flow ensures that the depleted gas about to leave the column encounters fresh liquid, the best possible absorbent. Near the bottom of the column, the liquid already contains much dissolved gas but it encounters the richest gas and further transfer is possible.

Theory of operation-Flooding & pressure drop

• The pressure losses accompanying the flow of fluids through packed columns are caused by simultaneous kinetic and viscous energy losses. The essential factors determining the energy loss, i.e. pressure drop, in packed columns are:

• 1. Rate of fluid flow • 2. Viscosity and density of the fluid • 3. Closeness and orientation of packing • 4. Size shape and surface of the particles • The first two variables concern the fluid, while the last two

the solids.

Theory of operation: Flooding & pressure drop

• When an inlet gas flow rate is so high that it interferes with the downward flow of the solvent liquid a phenomenon known as flooding occurs and may cause an upward flow of the liquid through the tower. Most absorbers are designed to operate at no more than 70% of the maximum gas velocity that can cause flooding.

• Items that may also lead to flooding include low liquid circulation rates and small diameter towers to name a few.

• Pressure drop across a packed bed tower is a common way of determining if flooding is occurring. It is also an effective way of determining if something else has gone wrong inside of the absorber.

• A large pressure drop across a packed bed could also indicate plugging in the bed and/or deterioration of the packing itself. A high pressure drop can also indicate a variety of other abnormalities with the absorber that can ultimately result in a low gas removal efficiency.

PACKED COLUMN DESIGN

• Numerous methods of design are available in literature

• The one adopted here is due to Nguyen (1978)• An Excel spreadsheet and Matlab programmes

will be used for the design• Equations required:- (1)

995851.3/ln9729.0

/ln083472.0)ln( 221

GB

GBGAY

PACKED COLUMN DESIGN

• Where A= Specific surface of packing material• G=Superficial mass velocity of gas• A1=AμL

0.2/(gcε3ρGρL) (2)

• B=L (ρG/ρL)0.5 (3)• The 1st derivative of (1) is •

• The column diameter is then calculated as

)4(/ln/1166944.0/10271.1' GBGGY

)5..(..................../59577.1 5.0GWD

FLOODING MASS VELOCITY

• The calculation of flooding velocity and diameter using correlation charts is prone to error and takes time.

• Excel spreadsheet and Matlab programme will be used instead.• The Excel and Matlab programmes are given in the following page

and next using the data given in the spreadsheet (Nguyen, 1978).

FLOODING MASS VELOCITY CHART

n G F(G) F'(G)0 10 -4.98038 0.0740544 Table A1: Excel prgramme for calculating1 77.253035 -3.11765 0.0140041 packed tower diameter2 299.876416 -1.49679 0.00436273 642.960102 -0.45038 0.00223284 844.670898 -0.05244 0.00175355 874.576618 -0.00081 0.00170026 875.050643 -1.9E-07 0.00169947 875.050757 -1.1E-14 0.00169948 875.050757 0 0.0016994

INPUT DATA REQUIRED:SPECIFIC SURFACE AREA(FT 2̂/FT 3̂) ASUPERFICIAL LIQUID RATE,L(Ib/hrft 2̂) LWEIGHT FLOW RATE OF GAS,W(Ib/hr) WVISCOSITY OF LIQUID,V(Cp) VFRACTION OF FREE VOID SPACE IN PACKING,E EDENSITY OF GAS,RhoG(Ib/ft 3̂) RhoGDENSITY OF LIQUID,RhoL(Ib/ft 3̂) RhoL

THESE CALCULATIONS ARE REQUIRED FOR THE ITERATION:A1B

DIAMETER OF PACKED TOWER ft D

OGHENEOVO EMEMERURAI WRPC, EKPAN

Matlab Programme

• Table A.2: Matlab M-file for calculating flooding rate in a packed column

• function y=absorption(G)• A=51;L=1516;W=1800;V=33.9;E=.683;RHOG=0.075;RHOL=55.66;• A1=(A*V^0.2)/(417E6*RHOL*RHOG*E^3);• B=L*(RHOG/RHOL)^0.5;• y=log(A1*G^2)+0.083472*(log(B/G))^2+0.9729*log(B/G)

+3.99585;• %Type the following in the command window:• %options = optimset('Display', 'iter'); % Turn on Display• %G=fzero(@absorption,875,options);

APPLICATION OF SPREADSHEET AND MATLAB PROGRAMMES

• Application of the spreadsheet gives the flooding mass velocity as 875.05 Ib/hr/ft2 as given in Cell B10. The Matlab programme also gives the same value of G.

• The diameter of the column is then determined as shown in Cell G25 as 2.288ft.

CONCLUSION

• An Excel programme and Matlab programme were written for the calculation of the flooding mass velocity of a packed absorption tower.

• The results of the two programmes gave the same value of flooding mass velocity of 875Ib/hr.ft^2.

• The diameter of the column was then determined as 2.288ft.• The programmes can be used to determine the operating

mass velocity of gas since the diameter of an operating column is known.

• The operating mass velocity is always less than the design and is stated to be 50-70% of the design.

REFERENCES

• Nguyen, H.X. (1978),Computer programme Expedites Packed Tower Design,Chemical Engineering November 26 PP181-184

• W.L Badger and J.T Banchero.Introduction to Chemical Engineering,McGraw-Hill Newyork,1955,P427.

• Stroud K.A. (2003). Advanced Engineering Mathematics, 4th edition, Palgrave Macmillan,U.K pp14-19.