the promoter e ect of piperazine on the removal of carbon ... · the promoter e ect of piperazine...

The promoter effect ofpiperazine on the removal of

carbon dioxide

by Rob Lensen

7th January 2004

Summary

Carbon dioxide, which falls into the category of acid gases (as does hydrogen sulfide,for example) is commonly found in natural gas streams at levels as high as 80%.In combination with water, it is highly corrosive and rapidly destroys pipelinesand equipment unless it is partially removed or exotic and expensive constructionmaterials are used.

Removal of CO2 from gases can be split up into two different methods. Firstly, theremoval with alkanolamines by which the CO2 is removed by absorption reactions.The second method is the removal of CO2 with alkaline salts, for example sodium-or potassium carbonate. The major processes are based on aqueous solutions ofsodium and potassium compounds.

When alkanolamines are used for CO2 removal there are three main industry species.Formerly monoethanolamine (MEA) was used. But MEA is rapidly displaced bymore efficient systems because of the corrosive properties and high heat of reactionwith CO2 . Diethanolamine (DEA) is used for the treatment of refinery gases withCOS and CS2. DEA is less reactive with COS and CS2 than MEA and also a lowervapour pressure is needed. The disadvantages of DEA treatment are that vacuumdistillary is necessary and therefore difficult. Methyldiethanolamine (MDEA) hasbecome an important alkanolamine because of the low energy requirement, highcapacity and high stability. The disadvantage the low rate of reaction with CO2 .The rate of the reaction can be increased by using promoters, without diminishingthe MDEA advantages.

In the major processes based on alkaline salts aqueous solutions of sodium and po-tassium compounds are used. The hot potassium carbonate process is effectivelyused in many ammonia, hydrogen, ethylene oxide and natural gas plants. To im-prove CO2 absorption mass transfer and to inhibit corrosion, proprietary activatorsand inhibitors are added. These systems are known as ’activated hot potassiumcarbonate (AHPC) systems’.

To improve the CO2 removal processes different promoters are used. In 1980 piper-azine (PZ) was found as an effective promotor for alkanolamine processes. Severalresearch was done to the reaction mechanism and reaction kinetics of piperazinewith MDEA. It is proposed that the reaction of piperazine with MDEA can beconsidered as rapid pseudo first order with respect to tertiary amine. The reactioncan be considered second order but the pH of MDEA-PZ systems is never low enoughfor a second order reaction. A kinetics model that fits the expirimental data veryaccurate is not found yet. The combined model off instantanous piperazine modeland pseudo-first-oder fits the experimental data most accurate. When the effect ofpiperazine is compared to other known promoters it is shown that piperazine is amore effective promotor for alkanolamines.

3

The most recent discovery is the use of piperazine as promotor for aqueous po-tassium carbonate systems. It is shown that the addition of an amine increases theabsorption rate dramaticly. When piperazine is added the rate behavior of aqueouspotassium carbonate systems approaches that of 5M MEA at both 40◦C and 60◦C .At a rich loading, aqueous potassium carbonate systems are compareable withMDEA-PZ systems.

The overall conclusion from the data presented in this thesis is that piperazine is aneffective promotor in combination with MDEA and aqueous potassium carbonate.For usage on an industrial level further research is however needed.

Contents

Summary 2

1 Introduction 6

2 Overview of the current processes for CO2 removal 72.1 Procedures with alkanolamines . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Reaction mechanism of alkanolamines . . . . . . . . . . . . . 72.1.2 Advantages and disadvantages of several alkanolamines . . . 9

2.2 CO2 removal with alkaline salts . . . . . . . . . . . . . . . . . . . . . 92.3 Promoters and/or activators used in CO2 absorption . . . . . . . . . 10

3 Piperazine as activator for alkanolamines 123.1 Chemical data of piperazine . . . . . . . . . . . . . . . . . . . . . . . 123.2 Reaction mechanism with MDEA and PZ . . . . . . . . . . . . . . . 123.3 Reaction kinetics with PZ . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3.1 Kinetics with pseudo first-order model . . . . . . . . . . . . . 143.3.2 Kinetics with other models . . . . . . . . . . . . . . . . . . . 16

3.4 Comparison of PZ with other promoters . . . . . . . . . . . . . . . . 16

4 Piperazine as activator for hot carbonate solutions 204.1 Absorption and desorption of CO2 from hot carbonate solutions . . . 20

4.1.1 Kinetics with un-promoted carbonate solutions . . . . . . . . 214.2 Absorption with promoted carbonate solutions . . . . . . . . . . . . 21

4.2.1 Kinetics with promoted carbonate solutions . . . . . . . . . . 214.3 Results with piperazine as promotor . . . . . . . . . . . . . . . . . . 23

5 Conclusions 26

Bibliography 27

List of Figures

2.1 Structure of the alkanolamines . . . . . . . . . . . . . . . . . . . . . 7

3.1 Molecular structures of piperazine species . . . . . . . . . . . . . . . 133.2 Enhancement factors for 0.1M PZ / 4.2M MDEA at 40◦C [9] . . . . 153.3 Enhancement factors for 0.6M PZ / 4.0M MDEA at 40◦C [9] . . . . 173.4 Comparable absorption rates of different aqueous amine systems at

40◦C , as function of CO2 concentration [18]. . . . . . . . . . . . . . . 173.5 Comparison of PZ/MDEA blends to conventional blends [9] . . . . . 183.6 Effect of piperazine on apparent reaction rate constants of the reac-

tion of CO2 with aqueous solutions AMP at 40◦C [32] . . . . . . . . . 18

4.1 Comparison of promoted K2CO3 solutions . . . . . . . . . . . . . . . 244.2 CO2 heat of absorption in K2CO3/PZ . . . . . . . . . . . . . . . . . 24

Chapter 1

Introduction

Carbon dioxide, which falls into the category of acid gases (as does hydrogen sulfide,for example) is commonly found in natural gas streams at levels as high as 80%.In combination with water, it is highly corrosive and rapidly destroys pipelinesand equipment unless it is partially removed or exotic and expensive constructionmaterials are used. Carbon dioxide also reduces the heating value of a natural gasstream and wastes pipeline capacity. In Liquified Natural Gas (LNG) plants, CO2

must be removed to prevent freezing in the low-temperature chillers.

Also to diminish the threat of a rapidly changing climate, emissions of CO2 shouldbe reduced. One way to reduce these emissions is CO2 removal.

Aqueous alkanolamine solutions are the most widely used solvents for the acid gasabsorption process. Isaacs et al. [20] reported that aqueous solutions ofmonoethanolamine (MEA) have the properties of high reactivity, low solvent cost,ease of reclamation, and low absorption of hydrocarbons. Aqueous solutions ofN-methyldiethanolamine (MDEA) were found to be the attractive solution for thelower regeneration energy. MDEA is most used and described by Kohl et al.[22].

Chapter 2 gives an overview of the current processes for CO2 removal and the possib-ility of promoters. The next chapter contains information about the use of piperazineas a promotor for alkanolamines. The recent discovery to use piperazine as a pro-motor for hot potasium carbamate systems will be discussed in chapter 4. In thelast chapter the use of piperazine is discussed and conclusions are drawn from thegathered information.

Chapter 2

Overview of the current processes for CO2 removal

Removal of CO2 from gases can be split up into two different methods. Firstly, theremoval with alkanolamines by which the CO2 is removed by absorption reactions.Depending on the process requirements, several options for alkanolamine basedtreating solvents with varying compositions of solutions have been proposed [22].These options can be classified into four groups. 1) Amine-Water, 2) Amine-Water-Organic Solvent, 3) Amine promoted Carbonate Processes, and 4) Amine mixtures-Water/Organic Solvent. The second method is the removal of CO2 with alkalinesalts, for example sodium- or potassium carbonate. The major processes are basedon aqueous solutions of sodium and potassium compounds.

2.1 Procedures with alkanolamines

Bottoms [11] firstly described the absorption of CO2 by tri-ethanolamine (TEA).His patent was used for early gas-treating plants. Further research showed thatother alkanolamines could absorb CO2 as well. The most important and used al-kanolamines, shown in figure 2.1, are:

a. monoethanolamine (MEA)b. diethanolamine (DEA)c. methyldiethnolamine (MDEA)

NH2

C CHO

(a) MEA

ppppppppppppppppppppppppppppppppppppppppppppppppp

pppppppppppppppppppppppppppppppppppppppppppppppp

C C

NH

CC

HO

HO

(b) DEA

pppppppppppppppppppppppppppppppppppp

pppppppppppppppppppppppppppppppppppp

CH3

C C

CCHO

HO

(c) MDEA

Figure 2.1: Structure of the alkanolamines

2.1.1 Reaction mechanism of alkanolamines

To understand and clarify the absorption mechanism of CO2 by alkanolamines therehas to be discriminated between the primary, secondary, and tertiary amines.

Chapter 2. Overview of the current processes for CO2 removal 8

The mechanism of the primary and secondary alkanolamines MEA and DEA canbe represented as the general equations (2.1) and (2.2).

The overall forward reaction between CO2 and primary and secondary alkanolam-ines usually has been represented as:

CO2 + R1R2NH � R1R2NCOOH (2.1)

R1R2NCOOH + R1R2NH � R1R2NCOO− + R1R2NH+

2 (2.2)

The first step is bimolecular, second-order, and rate determining, while the secondstep was supposed to take place instantaneously. However, this scheme is a sub-stantial simplification for the reaction mechanism that actually occurs. Since 1960a large number of studies on the reaction between CO2 and alkanolamines in aqueoussolutions have been presented. These studies were reviewed by Blauwhoff et al. [10].From the results that have been obtained it can be concluded that only for MEAa general agreement exists on both the reaction order and the value of the kineticconstant. Versteegh and Oyevaar [33] have shown for the alkanolamine DEA thatthe reaction with CO2 occurs via the zwitterion type mechanism. For this amine nogenerally valid, unique reaction order can be represented. For other primary andsecondary alkanolamines overall reaction orders varying between two and three wereobtained, both for aqueous and non-aqueous solutions. Therefore the mechanism ofthe reaction between CO2 and alkanolamines is not as simple and straightforwardas suggested by the equations (2.1) + (2.2), even for MEA. Danckwerts [16] reintro-duced a reaction mechanism proposed originally by Caplow [12] which described thereaction between CO2 and alkanolamines via the formation of a zwitterion followedby the removal of a proton by a base, B:

CO2 + R1R2NH � R1R2N + HCOO− (2.3)

R1R2N + HCOO− + B � R1R2NCOO− + BH+ (2.4)

Research by Donaldson and Nguyen [17] has shown that for a pH < 11 this reactioncan be described with the base catalysis of the CO2 hydration:

CO2 + R1R2R3N + H2O � R1R2R3NH+ + HCO3− (2.5)

Versteeg and Van Swaaij [34] and Benitez-Garcia et al. [5] demonstrated that theabsorption of CO2 in aqueous triethylamine solution is identical to alkanolamines.Therefore the observed reactivity of tertiary alkanolamines towards CO2 at lowerpH-values of the aqueous solution could not be attributed to the formation of mono-alkylcarbonate. Furthermore, according to the mechanism proposed by Donaldsonand Nguyen [17], no reaction should occur if CO2 is absorbed into a non-aqueoustertiary amine solution. Versteeg and Van Swaaij have shown that the absorptionrate of CO2 into MDEA-ethanol could be described completely as a physical ab-sorption. It was almost identical to the absorption of N2O, corrected for the dif-ferences in physical constants, in the same solution. Moreover, the total amount ofCO2 absorbed was nearly the same as the amount which can be physically dissolvedin this solution. They were able to ascribe the differences completely to the presenceof primary and secondary amine impurities.


From this result it is easy to conclude that in non-aqueous solutions no reaction,even alkylcarbonate formation, occurs between CO2 and tertiary amines. This is ingood agreement with the proposed reaction mechanism. The reaction of tertiaryamines does not occur in non-aqueous solutions.

2.1.2 Advantages and disadvantages of several alkanolamines

In the industry many different alkanolamines are used. To understand why a amineis used in a specific situation the differences need to be shown.

Formerly MEA was used in gastreating plants for CO2 removal. But MEA is rap-idly displaced by more efficient systems. However, for low concentrations of H2Sand CO2 it is prefered. This is especially for low pressure and maximum removalof H2S and CO2 . The advantages are the high alkalinity and easy recovery fromcontaminated solutions. On the other hand an irreversible product is formed withCOS and CS2, it is more corrosive, high heat of reaction with CO2 and also a highvapour pressure [22].

DEA is used for the treatment of refinery gases with COS and CS2. DEA is lessreactive with COS and CS2 than MEA and also a lower vapour pressure is needed.The disadvantages of DEA treatment are that vacuum distillary is necessary andtherefore difficult. And when a high content of CO2 is present, DEA isn’t a goodchoice because of the forming corrosive degradation products [22].

MDEA is another alkanolamine which can be used for the removal of CO2 . Theadvantages given by Appl et al. 1980 [1] are low energy requirement, high capacityand high stability, the disadvantage the low rate of reaction with CO2 . The rate ofthe reaction can be increased by using promoters, without diminishing the MDEAadvantages. The different promoters will be discussed in section 2.3.

2.2 CO2 removal with alkaline salts

As mentioned in the introduction, CO2 can also be removed from the gases by theuse of alkaline salts. These salts are sodium and potassium carbonate, phosphate,borate, arsenite and phenolate. The major processes are based on aqueous solu-tions of sodium and potassium compounds. The hot potassium carbonate processis effectively used in many ammonia, hydrogen, ethylene oxide and natural gasplants. To improve CO2 absorption mass transfer and to inhibit corrosion, propri-etary activators and inhibitors are added. These systems are known as ’activatedhot potassium carbonate (AHPC) systems’ [22]. Which type of activators are usedwill be discussed in section 2.3.

The accepted mechanism of CO2 absorption into water consists of two parallel mech-anisms:

1. Direct formation of HCO−

3

CO2 + OH−� HCO−

3 (fast) (2.6)

HCO−

3 + OH−� CO−

3 + H2O (instant) (2.7)


2. Reaction of CO2 with water followed by dissociation of carbonic acid

CO2 + H2O � H2CO3 (slow) (2.8)

H2CO3 + OH−� HCO−

3 + H2O (instant) (2.9)

According to Astarita [2] the predominant mechanism at pH > 10 is reaction (2.6)The reaction rate at 105◦C is not high enough to be considered instantaneous, Sav-age [30]

2.3 Promoters and/or activators used in CO2 absorption

Because of the low reaction rate of the CO2 removal by alkanolamines or alkalinesalts, promotors or activators are needed to improve the absorption process. Thefollowing compounds can be used to increase the reaction rate:

• Formaldehyde [22]• Methanol [22]• Phenol [22]• Ethanolamine [22]• Arsenious acid [21]• Glycine [28]• Hinderd amine [22]

The general mechanism for promoted solutions is proposed by Astarita [3]:

CO2 + promoter � intermediate (2.10)

intermediate + OH−� promoter + HCO−

3 (2.11)

The effect of the promotion can be quite well described in terms of a homogeneouscatalysis [3, 31]

In the ’Hot Potassium carbonate process’, of Benfield [13], the alkanolamine DEAis used as the activator of the process, but Bartoo [4] discovered a new organicpromotor. This new activator has a better absorption rate than DEA, at lowervapour pressure. Another advantage is the excellent chemical stability. The activatoris called ACT-1.

The Giammarco Vetrocoke [19] process uses glycine as an activator. This leads to anincrease of 18% plantload compared to DEA. In dual-activated (amine + glycine)systems a lower CO2 vapour pressure, a higher regenerator efficiency and a higherO2 absorption rate is detected, then when a mono activated solution is used.

Addition of arsenic trioxide to aqueous sodium or potassium carbonate solutionsresults in a marked increase in the rate of absorption and a desorption of carbondioxide, when it is compared with the conventional carbonate solutions. The reactionrate increases because the rate of hydration of carbon dioxide to carbonic acid isincreased. There is also a shift of the pH toward the acid side in the regenerationstep, resulting in a more complete expulsion of the absorbed carbon dioxide.

The addition of a primary or secondary amine to a tertiary amine has found wide-spread application in the absorption and removal of CO2 from process gases.


The success of these solvents is due to the high rate of reaction of the primary orsecondary amine with CO2 combined with the low heat of reaction of the tertiaryamine. By adding small amounts of the primary or secondary amine, a high rate ofabsorption is seen in the absorber, while a low energy of regeneration is required inthe stripper. One such blend of amines is piperazine (PZ) activated methyldieth-anolamine (MDEA). These solvents have been used successfully for high capacityCO2 removal in ammonia plants and are patented by BASF [1]. In the next chaptera more detailed description will be given of the use of piperazine.

Chapter 3

Piperazine as activator for alkanolamines

In this chapter the influence of piperazine (PZ) on the reaction of alkanolamineswith CO2 is described. PZ is most active as promotor when used in combinationwith MDEA, therefore the most attention is payed to articles concerning this com-bination.

3.1 Chemical data of piperazine

In table 3.1 shows an overview of the chemical data of PZ.

Synonyms: Piperazine Anhydrous, DiethylenediamineMolecular Formula: C4H10N2

Formula Weight: 86.13Registry number: 110-85-0Density: 146Melting point 108-112◦CBoiling point 145-146◦CFlash point 82◦C

Structure

N

N

Table 3.1: Chemical data of piperazine

PZ may be synthesised by, for example, reacting monoethanolamine with ammo-nia, or reacting ethylene oxide and NH3 and cyclising the ethanolamines therebyobtained [1]. The ability of a solvent to remove carbon dioxide is dictated by itsequilibrium solubility as well as mass transfer and chemical kinetics characteristics.In the next paragraph the kinetics are discussed.

3.2 Reaction mechanism with MDEA and PZ

As shown in paragraph 2.1.1 MDEA has a base catalytic effect on the CO2 hydrolyticreaction.

Chapter 3. Piperazine as activator for alkanolamines 13

Xu et al.[37] was the first who studied the reaction kinetics of PZ with MDEA. Inthis study it is proposed that the reaction can be considered as rapid pseudo firstorder with respect to R3N. Also the CO2 absorbed by free PZ can be transferredto MDEA rapidly with itself resumed. As seen in table 3.1 piperazine containstwo basic nitrogens and can theoretically react with 2 mol of CO2 . However, thesecond amine groups ability to bind a second CO2 can be neglected, which was firstreported by Liu et al. [24]. Bishnoi and Rochelle [8] proposed that the second amineis reactive, but the pH of these systems is never low enough to observe di-protonatedPZ. It is proposed by Xu et al. [36] that the following reactions occur in a solutionof PZ mixed with MDEA:

MDEA + H+� MDEAH+ (3.1)

MDEA + CO2 → MDEA − CO2 (3.2)

MEA − CO2 + H2O � MDEAH+ + HCO −

3 (3.3)

PZ + CO2 → PZ − CO2 (3.4)

PZ − CO2 + H2O � PZH+ + HCO −

3 (3.5)

PZ − CO ∗

2 + H2O � PZCOO− H3O+ (3.6)

PZ + H+� PZH+ (3.7)

PZ − CO2 + MDEA � MDEA − CO2 + PZ (3.8)

HCO −

3 � CO 2−

3 + H+ (3.9)

H2O � H+ + OH− (3.10)

The mentioned reactions are also shown by Bishnoi and Rochelle [9], but theypropose that also the following reactions take place:

PZCOOO− + H2O + CO2 � PZ(COO−)2 + H3O+ (3.11)

MDEA + PZOO− + CO2 � PZ(COO−)2 + MDEAH+ (3.12)

In Figure 3.1 the piperazine species are shown that occur in the reactions.

Figure 3.1: Molecular structures of piperazine species

Now the reaction mechanism is known, the equilibrium constants can be calculated.These equilibrium constants are shown by Bishnoi and Rochelle [9] and Xu et al.[36].


The difference between the two articles is that Bishnoi and Rochelle have equilibriumconstants for di-protonated piperazine. To calculate the total conversion of MDEAto MDEAH+, PZ to PZCOO− and PZ to PZH+ the species balance has to be made.The overall MDEA balance is proposed as [9, 36]:

C◦

MDEA+ = CMDEA + CMDEAH+ (3.13)

Bishnoi and Rochelle have a slightly different PZ balance, because they think di-protonated PZ also exists in the reaction. The difference is viewed in bold in equa-tion (3.14):

C◦

PZ = CPZ + CPZH+ + CPZCOO−+ CH+P ZCOO− + CP Z(COO−)2 (3.14)

Also the overall CO2 balance from Bishnoi and Rochelle differs from Xu et al.

CHCO−

3

+ CCO2−

3

+ CCO2+ CPZCOO−+ CH+P ZCOO− + 2CP Z(COO−)2

= y(C◦

MDEA + C◦

PZ)

(3.15)

The influence of the extra parameters on the results is negligible at low loading,but at high loading these parameters become more important. Without reactions(3.11) and (3.12) the data was most of the time underpredicted for about 40%. [9].

3.3 Reaction kinetics with PZ

There are serveral articles describing the method to calculate the reaction kinetics ofPZ with MDEA [8, 9, 36, 37, 38]. Until the article of Bishnoi [9] the overall reactionwas proposed as pseudo first-order. Bishnoi proposed that pseudo first-order modelis a good assumption for MDEA solutions since the reaction of MDEA is slowenough that no significant depletion of MDEA occurs at the interface. The pseudofirst-order curve for PZ/MDEA blends occurs only true at low loading, therefore thekinetics model is changed to use a second-order rate. There is just one article thatproposes the use of a second-order model therefore the pseudo first-order model isalso shown.

3.3.1 Kinetics with pseudo first-order model

For low loading the pseudo first-oder model is used. This assumes the concentrationof the amine to be uniform across the cross section of the liquid boundary layer.

When this assumption is adopted the reaction represented in (2.1) is the dominantreaction of absorption of CO2 into activated MDEA solution. Simultaneously, theactivator PZ may react with CO2 in liquid film to form an intermediate as:

R′(NH)2 + 2CO2 → R′(NHCOO)2 (3.16)

This reaction is rapid and runs parallel with reaction (2.1). The hydrolytic reactionof R′(NHCOO)2 also takes place in equilibrium in the liquid phase as:

R′(NHCOO)2 + 2H2O R′(NH+2 ) + 2HCO−

3 (3.17)


If these reactions are taken into consideration the reaction rate has been proposedas [37, 38]:

r = (k2Cam + kpCp)(pCO2− p∗CO2

) (3.18)

This expression assumes that the conversion of MDEA and PZ is constant in theliquid film.

It is shown by Savage et al. [30] that chemical absorption theory can be applied tochemical desorption in a fast reaction regime. Therefore Xu et al. [37, 38] proposedthat the chemical absorption and desorption rate for activated MDEA solutions canbe expessed as:

NCO2= HCO2

√

DCO2(k2Cam + kpCp)(pCO2

− pCO2∗) (3.19)

NCO2= HCO2

√

DCO2(k2Cam + kpCp)(pCO2

∗ −pCO2) (3.20)

This is only true when the partial pressure of CO2 is not very high and the freeconcentration of MDEA is not very low. It is claimed by Bishnoi et al. [8] thatthe results of Xu et al. [37] are not valid, because of the high partial pressureof CO2 . Therefore the pseudo-first-order assumption breaks down, the reactionoccurs in a region where the piperazine concentration is depleted at the interface.In Figure 3.2 the results of Xu et al. are displayed. Bishnoi [9] proposes that themeasured enhancement was not significantly different from the enhancement inMDEA solutions, making it impossible to gain any information about the effectof PZ. The results are therefore best described as pseudo-first-order absorption intounpromoted MDEA.

Figure 3.2: Enhancement factors for 0.1M PZ / 4.2M MDEA at 40◦C [9]

Figure 3.2 also demonstrates the experimental error of the Xu et al. data [37]. It’sunreasonable to expect that PZ has a negative effect on the enhancement factor.This negative effect is seen when the loading is above 0.3 mol.


3.3.2 Kinetics with other models

The kinetics model need to be changed, since the model data does not fit theexperimental data at higher loading. Bishnoi [9] reviewed the different models to fitthe data.

The following models are discussed by Bishnoi:

• Pseudo first-order (EPFO)Pseudo first-order model, discussed in section 3.3.1. Bishnoi used the expres-sion:

EPFO =

√

(∑

kAm[Am])DCO2

k 2l

(3.21)

• Model with instantanous reactions and small driving force(EGLBL,INST )The enhancement factor predicted by the simple model of instantaneous re-actions and small driving force isn’t a good approximation of the data.

EGLBL,INST =k◦l, PROD

k ◦

l,CO2

[Am]T HCO2

∂P ∗

CO2/∂α

(3.22)

• Model with only instantaneous reactions for PZ (EPZ,INST )Good approximation is obtained at high loading if only the PZ reactions areconsidered instantaneous. This is done by increasing all carbamate and di-carbamate formation rate constants to very large numbers.

• Combined model off instantanous PZ model and pseudo-first-order(EPZ,INST + EPFO)The pseudo first-order model and the instantaneous PZ model can also becombined. As seen in Figure 3.3 the results of this model are quite accurate.This model can be expressed as follows:

E =

1

1

EPZ,INST+

1

EPFO

(3.23)

The results of this review are displayed in Figure 3.3.

With a model that fits the experimental data the real effect of piperazine can befound. In next section piperazine willl be compared to other promoters.

3.4 Comparison of PZ with other promoters

To conclude if piperazine is an effective promoter for carbon dioxide removal theeffect of piperazine needs to be compared to other promoters.


Figure 3.3: Enhancement factors for 0.6M PZ / 4.0M MDEA at 40◦C [9]

A first rough comparison is done by Erga et al.[18]. Figure 3.4 gives rate data asfunction of the CO2 concentration for the primary amines MEA(25%), DGA(25%),KGl(4.0M), the secondary amine DEA(50%), the tertiary amine MDEA(50%), aswell as for the mixture of MDEA(50%)-piperazine(5%), and piperazine(5%) alone(% = weight-%). Since the mass transfer surface area are not strictly controlled,these rate data are just a guideline. Erga et al. [18] concluded that the MDEA-piperazine system can be a promising solvent for recovering CO2 .

Figure 3.4: Comparable absorption rates of different aqueous amine systems at40◦C , as function of CO2 concentration [18].

Later on Bishnoi [9] compared the MDEA-piperazine blend with DEA/MDEA andMEA/MDEA blends. It was seen that the piperazine blends enhances the perform-ance of MDEA by a factor of 50 at low loading. Even at high loading (0.5 molCO2 /mol amine), the piperazine blend can be up to a factor of 5 more effectivethen 50% MDEA. Piperazine is found to be a more effective promoter than MEA orDEA, especially at low loading. The high driving force is not seen to have a majoreffect on the predicted enhancement factor except at high loading.


The decrease in enhancement is due to the depletion of piperazine and piperazinecarbamate at the interface. An overview of the results is displayed in Figure 3.5 [9].

Figure 3.5: Comparison of PZ/MDEA blends to conventional blends [9]

Piperazine is not only an effective promoter in combination with MDEA. It isproposed by Seo [32] that the addition of piperazine to an aqueous mixture of2-Amino-2-methyl-1-propanol (AMP) increases the reaction rate constant remark-ably. The apparent rate coefficient increased with increasing piperazine concentra-tion. In Figure 3.6 the apparent reaction rate constants for the reaction of CO2 intoaqueous solutions of AMP and mixtures of AMP and piperazine at 40◦C is displayed.

As shown in Figure 3.6 the promotion effect of piperazine decreased at high AMPconcentration. It is proposed that the decrease of promoter effect is the result of thedecrease of the molar concentration of piperazine in the mixture of amine solution.

Figure 3.6: Effect of piperazine on apparent reaction rate constants of the reactionof CO2 with aqueous solutions AMP at 40◦C [32]

Not mentioned before is the use of piperazine as promoter in combination withMEA.

A recent study of Rochelle et al. [27] showed that piperazine can also used as


promoter for MEA systems. An addition of 0.6M PZ in 1.0M MEA increases therate by a factor 2 to 2.5 at 60◦C .

The relative effect of piperazine is practically independent of CO2 loading, exceptat very high loading. This is because PZCOO− is also a reactive species with CO2 .Only at very high loading (0.8-0.9), are both PZ and PZCOO− depleted. Fromthis data it may be concluded that piperazine is also promising promoter for MEAsystems.

Chapter 4

Piperazine as activator for hot carbonate solutions

The hot carbonate process for CO2 removal from gas streams is of interest because ofthe high efficiency it exhibts. This process usually operates at temperatures around100◦C . At this temperature chemical absorption and desorption takes place, andit is believed that the liquid side mass transfer rates are significantly enhanced bychemical reactions.

The most recent discovery is the use of piperazine as promotor for aqueous po-tassium carbonate systems. Because of this recent discovery there is only one un-published article [14] that describes that effects of piperazine on the absorption ofcarbon dioxide. In this chapter some data of this article of Cullinane [14] is used.

The reaction mechanism without piperazine is decribed in reaction (2.6). It is shownby serveral investigators [6, 7] that potassium carbonate has a low heat of regener-ation, but its rate of reaction is slow compared to amines.

4.1 Absorption and desorption of CO2 from hot carbonatesolutions

To understand the influence of piperazine the reaction kinetics without piperazineis described first.

The overall reaction can be described as:

CO2 + CO2−3 ↔ 2HCO−

3 (4.1)

In absorption the reaction (4.1) proceeds from left to right, and takes place essen-tially through the following sequence of elementary steps:

CO2 + OH−� HCO−

3 (4.2)

H2O � H+ + OH− (4.3)

CO2−

3 + H+� HCO−

3 (4.4)

Reaction (4.2) is rate controlling, and the reactions (4.3) and (4.4) are occuringeverywhere at equilibrium. Another mechanism proposed by Wall [35] known as thedirect reaction of CO2 with water is not be accounted. Because the contribution tothe overall reaction is negligible unless the pH of the liquid solution is very low.

Chapter 4. Piperazine as activator for hot carbonate solutions 21

It is proposed by Savage [30] that the kinetic constant can be calculated with thefilm theory equations. The results of this kinetic constant indicates that, at temper-atures as high as 105◦C , the rate of the chemical reaction is not large enough forthe reaction itself to be regarded as instantaneous. Therefore, even at the highesttempratures of industrial practice the possibility of rate promotion by additives isof significant interest.

4.1.1 Kinetics with un-promoted carbonate solutions

Since reaction (4.2) is the rate controlling step, the rate equation can be describedas [30]:

rOH = kOH [OH−][CO2] − kOH− [HCO−

3 ] (4.5)

Where kOH and k−OH are forward an backward rate constants of reaction (4.2). Atequilibrium conditions equation (4.5) leads to:

k−OH [HCO−

3 ] = kOH [OH−][CO2]e (4.6)

The equilibrium concentration of CO2 is defined as [CO2]e.

The expression for the reverse reaction (4.2) in equation (4.6) has been evaluatedby considering conditions at equilibrium, but it is generally true, even when thesystem is not at equilibrium [15, 23]. Substituting equation (4.6) into (4.5) gives:

rOH = (kOH [OH−])([CO2] − [CO2]e) (4.7)

Carbonate bicarbonate system is a buffer solution, so the concentration of OH− ionin the solution near the surface of liquid is not significantly depleted by the absorbedCO2 . In this case, the carbon dioxide undergoes a pseudo-first order reaction andequation (4.7) may be rewritten as [3, 15]:

rOH = (k1([CO2] − [CO2]e) (4.8)

where k1 denotes apparent first-order rate constant.

4.2 Absorption with promoted carbonate solutions

Since there is just one article where piperazine is used as promoter for carbon dioxideabsorption, most of the kinetics are based on the theory of other amines. When theresults are compared there are not many differences so it is believed that piperazinehas a comparable reaction mechanism and the same kinetics.

4.2.1 Kinetics with promoted carbonate solutions

When a small amount of amine is added into a carbonate solution, the absorptionrate is enhanced greatly according the following reactions: [3]

CO2 + RR′NH RR′NCOOH (4.9)

RR′NCOOH + OH− HCO−

3 + RR′NH (4.10)


At higher temperatures, in the range of industrial operating conditions, the rateof reaction (4.10) increases significantly. Therefore the system is better representedby the homogeneous catalysis mechanism [3, 15] and reaction (4.9) is the rate-controlling step. It is proposed by Rahimpour et al. [25] that the rate equationof carbon dioxide with promoted hot potassium carbonate in liquid phase can bedescribed as pseudo first order:

r = (kOH [OH−] + kAm[Am])([CO2] − [CO2]e) = k([CO2] − [CO2]e) (4.11)

where k is the overall apparent first-order rate constant and is defined as:

k = (kOH [OH−] + kAm[Am]) (4.12)

When reaction (4.9) is not rate-controlling, but reaction (4.10) is rate-controllingthe rate equation can be described as follows [14]:

r =kf [CO2][Am]

1 +kr

∑

kb[B]

(4.13)

Equation (4.13) is the same as equation (4.11) when reaction (4.9) is rate-controlling,since the contribution of the bases,

∑

kb[B] , is large and the denominator reducesto a value of one. When deprotonation of the intermediate (reaction (4.12)) is rate-controlling ,

∑

kb[B], is small so that the denominator is taken into consideration.

In addition to chemical reaction, mass transfer becomes an important considerationin absorption processes. The mass transfer can be described by several theorys.As proposed by Rahimpour et al.[25] the mass transfer can be described with thepenetration surface renewal theory developed by Danckwerts [15]. The theory thatCullinane [14] uses is known as the eddy diffusivity theory. This theory has theadvantage that it is time-independent.

When the penetration surface renewal theory is used the mass transfer can bedescribed as:

NCO2= Ekl(CCO2i

− CCO2e) (4.14)

Where CCO2iis the concentration of carbon dioxide at the interface and CCO2e

is the equilibrium concentration of unreacted carbon dioxide in the bulk of liquidwhen the reverse reaction of carbon dioxide is appreciable. kl is liquid phase masstransfer coefficient and E is the enhancement factor and describes the mass transfercoupled by chemical reactions as[15]:

E =

√

1 +DCO2

k

k2L

(4.15)

Where k is defined by equation (4.12). The rate of absorption which is defined byequation (4.14), can be rewritten in terms of physical solubility of carbon dioxidein solution, H, in the reactive K2CO3 solution as:

NCO2= kLHE(PCO2i

− PCO2e) (4.16)

In the gas phase the mass transfer as:

NCO2= kgCO2

(PCO2−−PCO2i

) (4.17)


Where kgCO2is the gas phase mass transfer coefficient of carbon dioxide.

The total mass transfer is found by combining equations 4.16 and 4.17 and elimina-ting the interface partial pressure of carbon dioxide, PCO2i

. This gives the followingequation:

NCO2=

(

kgCO2kLEH

kgCO2+ kLEH

)

(PCO2− PCO2e

)

= KgCO2(PCO2

− PCO2e) (4.18)

Where KgCO2is overall gas phase mass transfer coefficient of carbon dioxide.

The eddy diffusivity theory for carbon dioxide can be described by Cullinane [14]:

∂

∂a

[

(DCO2+ εx2)

∂[CO2]

∂x

]

+ RCO2(4.19)

In this equation, the diffusion of CO2 is important near the gas-liquid interface. Thevalue of D/ε as it occurs in the solution is approximated by the diffusion coefficientof CO2 [14].

4.3 Results with piperazine as promotor

Rahimpour et al. [25] and Cullinane [14] proposes that the addition of an amine in-creases the absorption rate dramaticly. When piperazine is added the rate behaviorof this solvent approaches that of 5M MEA at both 40◦C and 60◦C . At a rich load-ing, both promoted K2CO3 solutions compare favourably with a MDEA/piperazineblend, as mentioned in section 3.4.

When the amine amount is increased beyond a specific amount, the promoter effectdecreases. Rahimpour et al. [25] proposes that this can be explained due a higherenhancement factor in the liquid phase, which is directly proportional to the overallmass transfer coefficient in the case of liquid-phase controlled mass transfer.

By increasing the amine concentration, the gas phase mass transfer is consideredthe major factor controlling the absorption process so the CO2 removal is unaffectedby increasing the promoter concentration.

Cullinane [14] compared the PZ-promoted K2CO3 with other promoters used inK2CO3 solutions diethanolamine (DEA) and an unspecified amine investigated bySatari and Savage [29]. The results are seen in Figure 4.1.

For this promoter comparison, CO2 loading was represented as the conversion ofCO2−

3 to HCO−

3 and Henry’s constant was estimated accounting only for the K2CO3

in solution. While each promoter improves the rates over un-promoted K2CO3 tosome degree, piperazine at 60◦C gives the best improvement. It is proposed that atthe temprature of 90◦C piperazine is much more favourably then the promoter DEAand the hindered amine. This behaviour can be partially attributed to improved ratebehaviour and partially to ’salting out’ CO2 at high temperatures and high ionicstrengths [14].


Figure 4.1: Comparison of promoted K2CO3 solutions

It is concluded by Cullinane [14] that the heat of absorption of CO2 increases withthe addition of PZ to aqueous potassium carbonate. With a comparable loading andpiperazine concentration, more potassium carbonate serves to decrease the heat ofabsorption only slightly, indicating that the amine is largely responsible for thereaction with CO2 . A decrease in loading results in a marked increase in the heatof absorption, most likely due to a difference in heats of absorption of piperazineand of piperazine dicarbamate. The results also suggest that promoted potassiumcarbamate solutions would possess a lower heat of absorption then comparableamine systems. An overview of the results is displayed in figure 4.2 [14].

Figure 4.2: CO2 heat of absorption in K2CO3/PZ


Proton NMR done by Cullinane suggests that piperazine carbamate is the dominantspecies at high loading. Consequently, it is responsible for most of the reaction rate.Given that piperazine reacts much faster than the carbamate, it can be concludedthat loading has a significant effect on absorption rates.

Chapter 5

Conclusions

The current processes for CO2 removal are used on a large scale. Most processesare based on well known data and therefore optimising the processes is not reallyan option anymore. Therefore the process need to be enhanced with promotors.One of the promotors that can be used is piperazine. Appl et al. [1] was the firstwho researched the influence of piperazine on the CO2 removal. It was shown thatpiperazine accelrates the absorption considerably.

The effect of piperazine on CO2 removal is researched in depth since 1992. The firstpublished results where from Xu et al. [37]. Later on also Bisnoi and Rochelle [8]researched the influence of piperazine. From the results that both research groupshave published it can be concluded that the MDEA-piperazine system is a promos-ing solvent for CO2 removal. It is concluded that a solvent of piperazine/MDEA(5 wt. %/45 wt. %) provides almost two orders of magnitude and more enhance-ment then 50 wt. % MDEA at low loading and one order of magnitude enhancementat moderate loading.

The use of piperazine as promotor for the CO2 absorption in aqueous potassiumcarbonate is recently found to be effective [14, 25]. Piperazine increases the absorp-tion rate of CO2 substantially. Current studies reveal that, coupled with the lowheat of absorption associated with aqueous K2CO3, the piperazine/K2CO3 systemcould potentially reduce energy costs associated with CO2 removal. More studies areneeded for this solvent over a broader range of industrially significant conditions.

Since most articles are from just two research groups (University of Texas and EastChina University) it is necessary that more research is done by different researchgroups. Also because the research group from the East China University is notalways that accurate with their results [26].

The overall conclusion from the data presented in this thesis is that piperazine is aneffective promotor in combination with MDEA and aqueous potassium carbonate.For usage on an industrial level further research is however needed.

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