133750394-04-absorbers-pdf

Absorbers

Introduct ion ............................................................... 110 Hydrocarbon Absorber Design ................................ 110 Hydrocarbon Absorbers, Optimization .................. 112 Inorganic Type ........................................................... 113

109

110 Rules of Thumb for Chemical Engineers

Introduction

A general study of absorption can be confusing since the calculation methods for the two major types are quite different. First, there is hydrocarbon absorption using a lean oil having hydrocarbon components much heavier than the component absorbed from the gas stream. These absorbers may or may not be reboiled. For designing these, one uses equilibrium vaporization constants (K values) similarly to distillation. Another similarity to distillation is the frequent use of fractionating trays instead of packing. Canned computer distillation programs usually include hydrocarbon absorber options.

The other major type is gas absorption of inorganic components in aqueous solutions. For this type design one uses mass transfer coefficients. Packed towers are used so often for this type that its discussion is often included under sections on packed towers. However, in this book it is included here.

Source

Branan, C. R., The Process Engineer's Pocket Handbook, Vol. 1, Gulf Publishing Co., 1976.

Hydrocarbon Absorber Design

Because of its similarity to distillation, many parts of this subject have already been covered, such as

1. Tray Efficiency 2. Tower Diameter Calculations 3. K Values

As for distillation, shortcut hand calculation methods exist, for hydrocarbon absorption. In distillation, relative volatility (~) values are generated from the K values. For hydrocarbon absorption the K values are used to gener- ate absorption and stripping factors. The 1947 Edmister 1 method using effective overall absorption and stripping factors and the well-known Edmister graphs are very popular for hand calculations. An excellent write-up on this and the Kremser-Brown-Sherwood methods are on pages 48-61 of Ludwig. 2

Edmister Method (1947). Briefly, the Edmister absorption method (1947) with a known rich gas going to a fixed tower is as follows:

1. Assume theoretical stages and operating temperature and pressure.

2. Knowing required key component recovery Ea, read Ae from Figure 1 at known theoretical trays n.

E a "- A

n+l e - m e

n+l (1) Ae -1

E S ~

M+I Se -Se

M+I Se -1

(2)

where

n = Number of theoretical stages in absorber m - Number of theoretical stages in stripper Ea = Fraction absorbed E s - Fraction stripped Ae = Effective absorption factor Se = Effective stripping factor

3. Assume a. Total mols absorbed b. Temperature rise of lean oil (normally 20-40~ c. Lean oil rate, mols/hr

4. Use Horton and Franklin's 3 relationship for tower balance in mols/hr. This is shown in Table 1.

5. Calculate L1/V1 and Ln/Vn. 6. Use Horton/Franklin method to estimate tower tem-

peratures. This is shown in Table 2. 7. Obtain top and bottom K values.

Table 1

Tower Balance

Quantity Symbol Equation

Rich gas entering at bot tom Vn. 1 Known Gas Absorbed AV Assumed Lean gas leaving absorber V1 V1 = Vn + 1 - AV Gas leaving bottom tray Vn Vn =Vn + I(V1/Vn + 1) 1/N

Gas leaving tray 2 from top V 2 V 2 - V l / (V l /V n + 1) 1/N

Lean oil L0 Known Liquid leaving top tray L1 L1 = L0 + V2 - V1 Liquid leaving bottom tray Ln Ln = Lo + AV

Absorbers 111

<:

q~

O~ le~

"I"1 O"O" = - ~ - l - - l * j

1 ,0 �84 .

0.95, 0.90 o esl 0.60 ' " 0.75 ~ 070 ~ 0.65 I

V, "q r~ F " -

0.40!

mmmmmm

mmm (se w=,Mcaim mm wmmm

: a)saeNipnimmme=ammeaammm

~ . ~ . I p _ | i l l l l n '

!

,"L.$-.'~ I I ]_j~o,~ l ~ ~ i : 1 1 1 1 IIII L J - J - ~ - - r ,

o.~5 1 ~ i ~ r ~ _ . . ~ i L.J--F.-T-'t'-I-N 1--i""i~-' I i o.=o . ~ . , ~ ~ 1 1 1 1 l I 1 I 1 1 1 1 1 o.os, ~ " F I l I1 1 1 i 1 I 1 l I I 1I 1 l I

~ 0.2 0.3 0.4 0.5 0.6 o.7 o.e 0.9 =.o i2 Values of Ae or Se

L4 1.6 1.8 2.0 3.0 4.0 5.0 6.0

Figure 1. This graph shows the abosrption and stripping factors, Ea and Es, versus effective values, Ae and S e (efficiency functions). (By permission, W. C. Edmister, Petroleum Engineer, September, 1947 Series to January, 1948.)

Table 2 Tower Temperatures

Temperature Symbol Equation

Rich gas inlet Tn.l

Lean oil To

Temperature rise AT

Bo t t om Tray T n

Top Tray T1

K n o w n

K n o w n

Assumed T n = Tn+ 1 + AT

(Vn+ 1 - V 2 ) T 1 = T n - AT

(Vn+ 1 - V l )

8. Calculate absorption factors for each component i at the top and bottom

ATi -- L 1 / K l i Vl (3)

ABi - L n / K n i Vn (4)

For stripping factors

STi -- V, K , i / L , (5)

SBi -- Vn K n i / L n (6)

9. Obtain Aei from

A e - lAB (AT + 1) + 0.2511/2 - 0 .5 (7)

Similarly

S e - [ST(S B "1" 1)+ 0.251 '/2 - 0 .5 (8)

10. Read Eai values from Figure 1. 11. Calculate mols of each component absorbed. 12. Compare to assumed total mols absorbed and

reassume lean oil rate if necessary.

Edmister Method (1957). Edmister has developed an improved procedure 4 that features equations combining absorption and stripping functions as follows:

V1 - ~)aVn+l + (1 -- ~ ) s )ko

L1 - l~sLm+, + (1 - ~)a )Vo

(Absorption Section) (9)

(Stripping Section) (10)

where

L1 = Liquid from bottom stripping tray Lm+l = Liquid to top stripping tray

* a -" 1 - Ea, fraction not absorbed 0s = 1 - Es, fraction not stripped

Vo = Vapor to bottom stripping tray

Other symbols are defined in Tables 1 and 2. Figure 1 and Equations 3-8 are used as before.

V~ and L1 are found from Equations 9 and 10. The improved procedure is better for rigorous solution of complicated absorber designs.

Lean 0il. The selection of lean oil for an absorber is an economic study. A light lean oil sustains relatively high lean oil loss, but has the advantage of high mols/gal com- pared to a heavier lean oil. The availability of a suitable


material has a large influence on the choice. A lean oil 3 carbon numbers heavier than the lightest component absorbed is close to optimum for some applications. In natural gas plant operations, however, the author gen- erally sees a lean oil heavier by about 10-14 carbon numbers.

Presaturators. A presaturator to provide lean oil/gas contact prior to feeding the lean oil into the tower can be a good way of getting more out of an older tower. Absorber tray efficiencies run notoriously low. A presaturator that achieves equilibrium can provide the equiva- lent of a theoretical tray. This can easily equal 3-4 actual trays. Some modem canned computer distillation/absorption programs provide a presaturator option.

Sources

1. Edmister, W. C., Petroleum Engineer, September, 1947 Series to January, 1948.

2. Ludwig, Applied Process Design For Chemical and Petrochemical Plants, 2nd Ed., Vol. 2, Gulf Publish- ing Company, 1979.

3. Horton, G. W. and Franklin, B., "Calculation of Absorber Performance and Design," Ind. Eng. Chem. 32, 1384, 1940.

4. Edmister, W. C., "Absorption and Stripping-factor Functions for Distillation Calculation by Manual- and Digital-computer Methods," A.L Ch.E. Journal, June 1957.

5. Fair, James R., "Sorption Processes for Gas Separa- tion," Chemical Engineering, July 14, 1969.

6. Zenz, E A., "Designing Gas-Absorption Towers," Chemical Engineering, November 13, 1972.

7. NGPSA Engineering Data Book, Natural Gas Proces- sors Suppliers Association, 9th ed., 1972.

8. Norton, Chemical Process Products, Norton Company, Chemical Process Products Division.

9. Treybal, R. E., Mass Transfer Operations, McGraw- Hill Book Co., Inc., New York, 1955.

10. Rousseau, R. W. and Staton, S. J., "Analyzing Chem- ical Absorbers and Strippers," Chemical Engineering, July 18, 1988.

11. Diab, S. and Maddox, R. N., "Absorption," Chemical Engineering, December 27, 1982.

12. Branan, C. R., The Process Engineer's Pocket Hand- book, Vol. 1, Gulf Publishing Co., 1976.

Hydrocarbon Absorbers, Optimization

This section is a companion to the section titled Fractionators-Optimization Techniques. In that section the Smith-Brinkley 1 method is recommended for optimization calculations and its use is detailed. This section gives similar equations for simple and reboiled absorbers.

For a simple absorber the Smith-Brinkley equation is for component i:

f _ (1- sN)+qs(SN -- S) 1 - S N+I

where

f i - (BXB/FXF)i Smi- K'iV'/L' Sni- KiV/L

For a reboiled absorber:

f _ (1-- anN-M) W qs( SnN-M - an )

(1 -- SnN-M) -+- hSnN-M (1 -- Sm M+I )

where hi- correlating factor; if the feed is mostly liquid, use

Equation 1 and if mostly vapor, Equation 2.

hi - (K~//Ki)(L/L')[(1 - S n)/(1 - S m )]i (1)

hi - (L/L')[(1- Sn ) / (1- Sm )]i (2)

Nomenclature

B - Bottoms total molar rate, or subscript for bottoms F - Feed total molar rate, or subscript for feed f i - Ratio of the molar rate of component i in the

bottoms to that in the feed Ki- Equilibrium constant of component i in top section

= y/x K'i-Equilibrium constant of component i in bottom

section- y/x L - Effective total molar liquid rate in top section

L ' -Effec t ive total molar liquid rate in bottom section

Absorbers 113

M = Total equilibrium stages below feed stage including reboiler

N = Total equilibrium stages including reboiler and partial condenser

qs = Fraction of the component in the lean oil of a simple or reboiled absorber

S = Overall stripping factor for component i Sm = Stripping factor for component i in bottom section Sn = Stripping factor for component i in top section V = Effective total molar vapor rate in top section

V' - Effective total molar vapor rate in bottom section X - Mol fraction in the liquid Y - Mol fraction in the vapor

Sources

1. Smith, B. E., Design of Equilibrium Stage Processes, McGraw-Hill, 1963.


Inorganic Type

Design of inorganic absorbers quite often involves a system whose major parameters are well defined such as system film control, mass transfer coefficient equations, etc. Ludwig 1 gives design data for certain well-known systems such as NH3-Air-H20, C12-H20, CO2 in alkaline solutions, etc. Likewise, data for commercially available packings is well documented such as packing factors, HETR HTU, etc.

Film Control. The designer needs to know whether his system is gas or liquid film controlling. For commercial processes this is known.

In general, an absorption is gas film controlling if essentially all resistance to mass transfer is in the gas film. This happens when the gas is quite soluble in, or reactive with, the liquid. Ludwig 1 gives a listing of film control for a number of commercial systems. If a system is essentially all gas or liquid film controlling, it is common prac- tice to calculate only the controlling mass transfer coefficient. Norton 2 states that for gas absorption, the gas mass transfer coefficient is usually used, and for stripping the liquid mass transfer coefficient is usually used.

Mass Transfer Coefficients. General equations for mass transfer coefficients are given in various references if specific system values are not available. These must, however, be used in conjunction with such things as packing effective interfacial areas and void fractions under operating conditions for the particular packing selected. It is usually easier to find KGA for the packing used with a specific system than effective interfacial area and operation void fraction. Packing manufacturers' data or references, such as Ludwig ~ can provide specific system KGA or KLA data. Tables 1-6 show typical KGA data for various systems and tower packings.

If KGA values are available for a known system, those of an unknown system can be approximated by

KCA (unknown)- KGA (known)(Dvunkn~ ~ Dvknown

where

KCA -- Gas film overall mass transfer lb mols/hr (ft 3) (ATM)

Dv -Diffusivi ty of solute in gas, ftZ]hr

coefficient,

Diffusivities. The simplest gas diffusivity relationship is the Gilliland relationship.

Dv - 0 .0069 T3/2 (1/MA + 1/MB ),/2 P(V~/3 + V~/3) 2

where

T = Absolute temperature, ~ MA, MB = Molecular weights of the two gases, A and B

P = Total pressure, ATM VA, VB = Molecular volumes of gases, cc/gm mol

Height of Overall Transfer Unit. Transfer unit heights are found as follows"

H o G -- Gm

KGAPAvR

H o L -- Lm

KLApL


Table 1 KGA For Various Systems 4

KGA Solute Gas Absorbent Liquid Lb mols/(hr �9 ft 3 �9 atm)

Br2 5% NaOH 5.0 Cl 2 Water 4.6 CI 2 8% NaOH 14.0 CO 2 4% NaOH 2.0 HBr Water 5.9

HCHO Water 5.9 HCI Water 19.0 HCN Water 5.9 HF Water 8.0 H2S 4% NaOH 5.9 NH 3 Water 17.0 SO2 Water 3.0 SO2 11% Na2CO3 12.0

Table 2 Relative KGA For Various Packings 4

Type of Packing Material Relative KGA

Super Intalox | Plastic 1.00 Intalox | Saddles Ceramic 0.94 Hy-Pak | Metal 1.11 Pall Rings Metal 1.06 Pall Rings Plastic 0.97 Maspac Plastic 1.00 Tellerettes Plastic 1.19 Raschig Rings Ceramic 0.78

Table 3 Overall Mass Transfer Coeff icient CO2/NaOH System 5

Metal Tower Packings

KGA Packing (Ib-mol/h �9 ft 3 �9 atm)

#25 IMTP | Packing 3.42 #40 IMTP | Packing 2.86 #50 IMTP | Packing 2.44 #70 IMTP | Packing 1.74 1 in. Pall Rings 3.10 1�89 in. Pall Rings 2,58 2 in. Pall Rings 2.18 3�89 in. Pall Rings 1.28 #1 Hy-Pak | Packing 2.89 #1�89 Hy-Pak | Packing 2.42 #2 Hy-Pak | Packing 2.06 #3 Hy-Pak | Packing 1.45

Table 4 Overall Mass Transfer Coeff icient CO2/NaOH System s

Plastic Tower Packings

KGA Packing (Ib-mol/h �9 ft 3 �9 atm)

#1 Super Intalox | Packing 2.80 #2 Super Intalox | Packing 1.92 #3 Super Intalox | Packing 1.23 Intalox | Snowflake | Packing 2.37 1 in, Pall Rings 2.64 1�89 in. Pall Rings 2.25 2 in. Pall Rings 2,09 3�89 in. Pall Rings 1.23


Ceramic Tower Packings

K~ Packing (Ib-mol/h �9 ft 3 �9 atm)

1 in. Intalox | Saddles 2.82 1�89 in. Intalox | Saddles 2.27 2 in. Intalox | Saddles 1.88 3 in. Intalox | Saddles 1.11 1 in. Raschig Rings 2.31 1�89 in, Raschig Rings 1.92 2 in. Raschig Rings 1.63 3 in. Raschig Rings 1.02


Structured Tower Packings

K~ Packing (Ib-mol/h * ft s * atm)

Intalox | Structured Packing 1T Intalox | Structured Packing 2T Intalox | Structured Packing 3T

4.52 3.80 2.76

where

HoG, HOL --Height of transfer unit based on overall gas or liquid film coefficients, ft

Gm, Lm - G a s or liquid mass velocity, lb mols/(hr) (ft 2) KGA, KLA-Gas or liquid mass transfer coefficients,

consistent units P A r R -" Average total pressure in tower, ATM

DE --Liquid density, lb/ft 3

Absorbers 115

Number of Transfer Units. For dilute solutions the number of transfer units NoG is obtained by

N o G --

Y1-Y2 ( Y - Y * ) I - ( Y - Y*):

In (Y - Y *)1 (Y-Y*)2

where

( Y - Y*) = Driving force, expressed as mol fractions Y = Mol fraction of one component (solute) at

any point in the gas phase Y* = Mol fraction gas phase composition in equi-

librium with a liquid composition, X

X - M o l fraction in the liquid at the same corre- sponding point in the system as Y

1, 2 - Inlet and outlet of the system, respectively

Sources

1. Ludwig, E. E., Process Design for Chemical and Petrochemical Plants, Vol. 2, Gulf Publishing Co., 1965.

2. Norton Chemical Process Products, Norton Company, Chemical Process Products Division.


4. Branan, C. R., The Process Engineer's Pocket Hand- book, Vol. 2, Gulf Publishing Co., Houston, Texas, 1983.

5. Strigle, R. E, Packed Tower Design and Applications, 2nd Ed., Gulf Publishing Co., Houston, Texas, 1994.

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