absorption of carbxvxcon dioxide into a mixed aqueous solution of diethanolamine and piperazine

8/11/2019 Absorption of Carbxvxcon Dioxide Into a Mixed Aqueous Solution of Diethanolamine and Piperazine

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Indian Journal of Chemical TechnologyVol. 17, November 2010, pp. 431-435

Absorption of carbon dioxide into a mixed aqueous solution of diethanolamine

and piperazine

M K Mondal

Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University,Varanasi 221 005, India

Email: [email protected]

Received 4 August 2009; revised 17 August 2010

The CO2loading in aqueous mixtures of diethanolamine (DEA) and piperazine(PZ), from a mixture of CO2and N2, hasbeen measured for total amine concentrations and mole ratios of PZ to total amine ranging from 2.0 to 3.0 M and 0.01 to

0.20, respectively, at 313.14 K and 15.199 kPa CO2 partial pressure. Measurements were made by a saturation method usinga laboratory scale bubble column. The results of CO2loading are expressed as XCO2(mole CO2/mole of total amine) for allexperimental runs. A model is given to predict the CO2loading in aqueous mixture of DEA and PZ. The model predictionshave been in good agreement with the experimental data of CO 2 loading in aqueous mixture of DEA and PZ with the

average deviation of 8.67.

Keywords: DEA, PZ, CO2Loading, Total amine, Bubble column

The removal of acidic gases (in particular carbon

dioxide and hydrogen sulphide) from a gas stream is

an area of industrial importance. Examples of suchstreams include natural gases, synthesis gases from

the gasification of coal and heavy oils, and tail gases

from sulphur plants and petroleum chemical plants.The removal of acid gases has a goal to increase the

industrial and commercial utility of the hydrocarbonstreams, reducing contaminant emissions to the

environment during the combustion of such streams,

to reduce the corrosion problems in equipment and

pipelines due to the presence of such acid gases, and

additionally to take advantage of these gases forapplications in other industrial processes, such as in

the sulphur production in the case of hydrogen

sulphide1. Due to their chemically active nature, these

acidic gases may be absorbed by a number of

different chemical and physical absorbents2

. Theremoval of acid gases from a gas stream is carried out

mainly by means of a chemical reaction rather than

only physical absorption in which different chemical

reactions occur between the acid gases and the

constituents of the aqueous solution3. Particularly, the

removal of carbon dioxide by using chemical

absorbents has been of great importance, since it was

found that the global warming effect is primarily due

to excessive discharge of carbon dioxide and

methane4. Aqueous alkanolamine solutions have been

extensively used for the removal of CO2 from gas

streams5. A wide variety of alkanolamines, such as

monoethanolamine (MEA), diethanolamine (DEA),di-2-propanolamine (DIPA) and N-

methyldiethanolamine (MDEA), has been used for

industrial gas treating processes6

. Aqueous MEAsolutions are the most frequently used absorbents

because of high reactivity to such chemicals as CO2.However, these solutions can also react with materials

in the reaction vessels, tubing lines, and several

process compartments. For this reason, highly MEA-

concentrated aqueous solutions should be avoided for

the CO2removal process.The use of amine blends may have the potential of

solving this problem. A blended amine solvent, which

is an aqueous blend of a primary or a secondary

amine with a tertiary or a hindered amine, combines

the higher equilibrium capacity of the tertiary orhindered amine, with the higher reaction rate of the

primary or secondary amine. Thus the use of blended

amine solvents, requiring lower circulation rates and

lower regeneration energy, can bring about

considerable improvement and great savings in

individual gas-treating processes7. The low vapour

pressure of DEA makes it suitable for low pressure

operations, as vaporization losses are quite negligible.

Besides DEA solution are in general, less corrosive

than MEA solutions. In view of this, DEA based


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INDIAN J. CHEM. TECHNOL., NOVEMBER 2010432

blends appear to be potential solvents for gas treatingprocesses. Xu et al.

8measured the solubility data for

CO2in 4.28 kmol/m3 MDEA with the Piperazine (PZ)

concentration ranging from 0 to 0.515 kmol/m

3

andCO2partial pressure ranging from 3.83 to 76.77 kPa.

Bishnoi and Rochelle9 showed that PZ has a large

effect on solubility when the ratio of total CO2to PZ

is less than unity.

Piperazine is an effective activator for an industrial

CO2removal process. However the absorption data of

CO2 in the aqueous blends of DEA with PZ is veryscarce. In this study, the absorption data of CO2 in

DEA-PZ-H2O solution was systematically determined

and represented by a simple model.

Experimental ProcedureMaterials

Reagent grade DEA was obtained from Sisco

Research Laboratory Pvt. Ltd., Mumbai with a purity

of 98%. PZ was obtained from SD Fine Chemical

Ltd., Mumbai with a purity of better than 99% and

was used without further purification. All solutions

were prepared with distilled water. The carbon

dioxide and nitrogen gases provided were of a

commercial grade with a purity of 99.5 mole%.

Apparatus and procedure

For the absorption of CO2into an aqueous blend of

DEA and PZ, a borosilicate glass bubble column was

used. The schematic diagram of experimental set-up

is shown in Fig. 1 to determine the experimental datafor this work. Carbon dioxide with a partial pressure

of 20.265 kPa from a gas cylinder is passed through

the gas rotameter at fixed flow rate and sent to thebubble column. The bubble column containing an

aqueous blend of DEA and PZ was submerged in a

constant temperature water bath. The temperature inthe bubble column was controlled within 0.1 K of

the desired level with a thermometer, and allmeasurements were done at atmospheric pressure.

The dimensions of various main units including

material of construction were reported elsewhere10

and the experimental conditions used in the presents

work are shown in Table 1. Before starting theexperiment, a constant temperature was attended

inside the bath to maintain the desired temperature

inside the content of the bubble column. After that the

main gas stream is slowly turned on and maintained

the minimum possible gas flow rate so that it was

bubbled through the liquid. Determination of the CO2

concentration in the gas phase was made for each run

with the help of microprocessor based CO2 analyzer

(UNIPHS 225 PM, United Phosphorous Limited,

Mumbai). The equilibrium state was assumed when

the outlet composition becomes equal to the inlet gas

composition. The equilibrium CO2 loading in the

liquid phase was determined by acidulating known

volume of the loaded liquid sample with 6 M HCl and

Fig. 1Experimental set-up for bubble column: 1- Gas cylinder (CO2and N2); 2- 2-Stage S.S. pressure regulator; 3- S.S. Valve; 4-Gasrotameter; 5-Gas mixing and pressure release chamber; 6- Bubble column; 7- Constant temperature water bath; 8- Glass Tee; 9- Moisture

trap column; 10- CO2analyzer; 11- Wet gas flow meter; 12- Flat bottom flask with absorbing solution

Table 1Experimental conditions used in the present work

CO2Partial pressure in inlet gas stream (kPa) 10.133-20.265Temperature of the liquid bed (K) 303.14-353.14Volume of the liquid (cm3) 500Height of the liquid bed (cm) 28.2

pH of the distilled water used 6.95

Mole ratio of PZ in total amine 0.01-0.20Total amine concentration (M) 1.0-4.0


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MONDAL: ABSORPTION OF CARBON DIOXIDE 433

measuring the volume of the evolved gas by aprecisely graduated gas burette. At a given

temperature and pressure, at least two liquid

equilibrium samples were taken in order tocheck reproducibility and the estimated error in

the measured data is about 0.5%. The temperatureof the liquid inside the bubble column was controlled

within 0.1 K upto 353.15 K. The total pressure

was measured for each run with an uncertainty

of 0.5 kPa upto 20.265 kPa.

Model development

The absorption of CO2into an aqueous blend of PZ

and DEA can be explained by a homogeneous

activation mechanism. The reaction of CO2 with PZ

can be regarded as the rapid pseudo-first order

reaction in parallel with that of CO2 with DEA.Piperazine contains two basic nitrogens and can

theoretically react with 2 mole of CO2. PZ could also

be protonated. Therefore, the effective free PZ,

although its concentration is very low, can transfer

CO2 to DEA as a homogeneous activator andobviously promote the CO2 absorption rate of

activation DEA aqueous solutions. In an aqueous

phase for the CO2-DEA-PZ-H2O system, the

following chemical equilibria are involved11

:

PZH+

PZ + H+

(1)

DEAH+ DEA + H+ (2)

H2O+CO2 H++ HCO-3 (3)

HCO-3 H

++ CO

2-3 (4)

H2O H++ OH- (5)

Experimental work in the present study is limited to

low partial pressure range, the gas phase is considered

to be ideal. In the low range of CO2partial pressure

the fugacity of CO2 becomes equal to its partialpressure and gas-liquid phase equilibria for CO2

may be described by Henrys law. In solution, the

equilibrium constants of the above mentioned

equilibrium reactions are:

K1 = CPZ CH+/ CPZH+ (6)

K2= CDEACH+/CDEAH+ (7)

K3= CH+CHCO3- /CCO2CH2O (8)

K4= CH+CCO32-/CHCO3

- (9)

K5= CH+COH-/CH2O (10)

The concentration of H+and OH-are rather low, so it

is reasonable to neglect their effects on the mass and

charge balance equations. Another assumption is that

all forms of the absorbed carbon dioxide are regardedas bicarbonate since the contents of CO3

2-and CO2are

also very low. Thus only three main dissociation

reactions are taken in consideration and are also

expressed as given by Eqs (6-8).

Mass and charge balances for the reacting speciescan be written as:

CPZ, total= CPZ+ CPZH+ (11)

CDEA, total= CDEA+ CDEAH+ (12)

XCO2= CHCO3_/CPZ, total+ CDEA, total (13)

CPZH++ CDEAH+= CHCO3

_ (14)

Gas-liquid equilibrium of CO2may be expressed as:

pCO2= HCCO2 (15)

The CO2 loading can be expressed as its partial

pressure by solving Eqs (6-8) and Eqs (11-15) and

given below:XCO2= 2K3pCO2/[2K3pCO2 + H {-b + (b

2 4ac)^(1/2)}]

(16)

where

a = (CPZ, total+ CDEA, total) (1 XCO2) (17)

b = (CPZ, totalK2+ CDEA, totalK1) (1 XCO2)

(CPZ, totalK1+ CDEA, totalK2) XCO2 (18)

c = - (CPZ, total+ CDEA, total) K1K2XCO2 (19)

The thermodynamic equilibrium constants and

Henrys law constant are taken from literature and

presented in Table 2.

Table 2Temperature dependence of the equilibrium constants and Henrys constant

Parameter Expression Reference

K1 ln K1= -11.53 4345.5/T Pagano et al.12

K2 ln K2= 0.0099 13.3373/T 4218.7 ln T Austgen et al.13

K3 ln K3= -241.828 + 29.825 104/T 1.485 108/T2 + 0.333 1011/T3 0.282 1013/T4 Kent and Eisenberg14

H lnH= 20.267 1.383 104/T+ 0.069 108/T2 0.016 1011/ T3+ 0.012 1013/ T4 Kent and Eisenberg14


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INDIAN J. CHEM. TECHNOL., NOVEMBER 2010434

Results and Discussion

Comparison of CO2 loading of present study with literature

data

In literature, a lot of amine blends had been workedout for measuring CO2 loading. Among all of

previous works, some of amine blends are comparedwith the experimental data of present study for same

experimental conditions such as partial pressure of

CO2in inlet gas stream, total concentration of amine

blend and mole ratio of an amine constituent in blend.

For comparison purpose, blends consisting either

DEA or PZ as one of the component has been used.

The CO2 loading comparisons of experimental dataof present study with the other blends are taken

at total amine concentrations of 2.0, 2.5 and 3.0 M,

respectively. The CO2loading comparisons are shown

in Figs 2 to 4. From Figs 2-4, it is seen that CO 2

loading is highest for DEA-PZ blend of present study

at all total concentration range 2.0-3.0 M with

experimental conditions of 313.14 K and 15.199 kPa

CO2 partial pressure in comparison with other

blends available in literature. Therefore, DEA-PZblend for present study is assumed to be superior to

other blends available in literature viz. DEA-AMP,

MDEA-PZ, DEA-MDEA, TIPA-PZ regarding CO2loading at experimental conditions of low temperature

and low CO2partial pressure.

Comparison between the experimental and model values

The model has been solved directly without

iterative calculation. In the model, CO2 loading

is expressed interms of partial pressure of CO2in inlet gas stream, Henrys law constant, equilibrium

constants of the reactions, and total concentrations

of DEA and PZ. Henrys constant and equilibrium

constants are function of temperature only, and

Fig. 4Comparison of CO2 loading in 3 M DEA-PZ blend withother 3 M blends available in literature

Fig. 5Comparison of the experimental solubility of CO2(XCO2)

and model values

Fig. 2Comparison of CO2 loading in 2 M DEA-PZ blend withother 2 M blends available in literature

Fig. 3Comparison of CO2loading in 2.5 M DEA-PZ blend with

other 2.5 M blends available in literature


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MONDAL: ABSORPTION OF CARBON DIOXIDE 435

can be calculated using equations as given inTable 2. From Eqs (17)-(19), the terms a, b and c

can be dependent on XCO2 only after knowing the

values of K1, K2 and total concentrations of DEAand PZ. Thereby, finally using Eq. (16) and with

the help of Henrys constant and equilibriumconstant K3 and pCO2, the values of XCO2 can be

obtained. A comparison of the experimental value

XCO2, exp and the model value XCO2, mod for the CO2

loading is shown in Fig. 5. As shown in Fig. 5, the

model value is in good agreement with theexperimental value with a mean deviation of 8.67%.

ConclusionIn this work, the CO2 loading has been measured

into mixed solutions of DEA and PZ within total

concentration range 2.0-3.0 M and with mole

ratios of PZ to total amine of 0.01-0.20 at 313.14 K

and 15.199 kPa CO2 partial pressure. CO2 loading

in the concentration range 2.0-3.0 M of DEA-PZ

blends of present study has been found to be

superior to other blends such as DEA-AMP,

MDEA-PZ, DEA-MDEA, TIPA PZ available

in literature. The data obtained from model is

agreed well with the experimental data of CO2

loading. The average deviation between the

calculated and experimental CO2 loading data

is 8.67%.

AcknowledgementAuthor gratefully acknowledges Banaras Hindu

University for providing the necessary help for

carrying out the present work.

NomenclatureCi = concentration of solute i, kmol/m

3

H = Henrys law constant, kPa.m3/kmol

K1 K5 = equilibrium constants

pCO2 = CO2partial pressure, kPaXCO2 = CO2loading, mole of CO2/mole of total amine

Subscriptsi = species or component

exp = experimental

mod = model

Amine abbreviationsAMP = aminomethylpropanol

DEA = diethanolamine

MDEA = methyldiethanolamine

PZ = piperazine

TIPA = triisopropanolamine

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absorption of carbxvxcon dioxide into a mixed aqueous solution of diethanolamine and piperazine

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