The promoter effect of piperazine on the removal of carbon dioxide

Transcription

1 The promoter effect of piperazine on the removal of carbon dioxide by Rob Lensen 7th January 2004

2 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 pipelines and equipment unless it is partially removed or exotic and expensive construction materials are used. Removal of CO 2 from gases can be split up into two different methods. Firstly, the removal with alkanolamines by which the CO 2 is removed by absorption reactions. The second method is the removal of CO 2 with alkaline salts, for example sodiumor potassium carbonate. The major processes are based on aqueous solutions of sodium and potassium compounds. When alkanolamines are used for CO 2 removal there are three main industry species. Formerly monoethanolamine (MEA) was used. But MEA is rapidly displaced by more efficient systems because of the corrosive properties and high heat of reaction with CO 2. Diethanolamine (DEA) is used for the treatment of refinery gases with COS and CS 2. DEA is less reactive with COS and CS 2 than MEA and also a lower vapour pressure is needed. The disadvantages of DEA treatment are that vacuum distillary is necessary and therefore difficult. Methyldiethanolamine (MDEA) has become an important alkanolamine because of the low energy requirement, high capacity and high stability. The disadvantage the low rate of reaction with CO 2. The rate of the reaction can be increased by using promoters, without diminishing the MDEA advantages. In the major processes based on alkaline salts aqueous solutions of sodium and potassium compounds are used. The hot potassium carbonate process is effectively used in many ammonia, hydrogen, ethylene oxide and natural gas plants. To improve CO 2 absorption mass transfer and to inhibit corrosion, proprietary activators and inhibitors are added. These systems are known as activated hot potassium carbonate (AHPC) systems. To improve the CO 2 removal processes different promoters are used. In 1980 piperazine (PZ) was found as an effective promotor for alkanolamine processes. Several research was done to the reaction mechanism and reaction kinetics of piperazine with MDEA. It is proposed that the reaction of piperazine with MDEA can be considered as rapid pseudo first order with respect to tertiary amine. The reaction can be considered second order but the ph of MDEA-PZ systems is never low enough for a second order reaction. A kinetics model that fits the expirimental data very accurate is not found yet. The combined model off instantanous piperazine model and pseudo-first-oder fits the experimental data most accurate. When the effect of piperazine is compared to other known promoters it is shown that piperazine is a more effective promotor for alkanolamines.

3 3 The most recent discovery is the use of piperazine as promotor for aqueous potassium carbonate systems. It is shown that the addition of an amine increases the absorption rate dramaticly. When piperazine is added the rate behavior of aqueous potassium carbonate systems approaches that of 5M MEA at both 40 C and 60 C. At a rich loading, aqueous potassium carbonate systems are compareable with MDEA-PZ systems. The overall conclusion from the data presented in this thesis is that piperazine is an effective promotor in combination with MDEA and aqueous potassium carbonate. For usage on an industrial level further research is however needed.

4 Contents Summary 2 1 Introduction 6 2 Overview of the current processes for CO 2 removal Procedures with alkanolamines Reaction mechanism of alkanolamines Advantages and disadvantages of several alkanolamines CO 2 removal with alkaline salts Promoters and/or activators used in CO 2 absorption Piperazine as activator for alkanolamines Chemical data of piperazine Reaction mechanism with MDEA and PZ Reaction kinetics with PZ Kinetics with pseudo first-order model Kinetics with other models Comparison of PZ with other promoters Piperazine as activator for hot carbonate solutions Absorption and desorption of CO 2 from hot carbonate solutions Kinetics with un-promoted carbonate solutions Absorption with promoted carbonate solutions Kinetics with promoted carbonate solutions Results with piperazine as promotor Conclusions 26 Bibliography 27

5 List of Figures 2.1 Structure of the alkanolamines Molecular structures of piperazine species Enhancement factors for 0.1M PZ / 4.2M MDEA at 40 C [9] Enhancement factors for 0.6M PZ / 4.0M MDEA at 40 C [9] Comparable absorption rates of different aqueous amine systems at 40 C, as function of CO 2 concentration [18] Comparison of PZ/MDEA blends to conventional blends [9] Effect of piperazine on apparent reaction rate constants of the reaction of CO 2 with aqueous solutions AMP at 40 C [32] Comparison of promoted K 2 CO 3 solutions CO 2 heat of absorption in K 2 CO 3 /PZ

6 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 pipelines and equipment unless it is partially removed or exotic and expensive construction materials are used. Carbon dioxide also reduces the heating value of a natural gas stream and wastes pipeline capacity. In Liquified Natural Gas (LNG) plants, CO 2 must be removed to prevent freezing in the low-temperature chillers. Also to diminish the threat of a rapidly changing climate, emissions of CO 2 should be reduced. One way to reduce these emissions is CO 2 removal. Aqueous alkanolamine solutions are the most widely used solvents for the acid gas absorption process. Isaacs et al. [20] reported that aqueous solutions of monoethanolamine (MEA) have the properties of high reactivity, low solvent cost, ease of reclamation, and low absorption of hydrocarbons. Aqueous solutions of N-methyldiethanolamine (MDEA) were found to be the attractive solution for the lower regeneration energy. MDEA is most used and described by Kohl et al.[22]. Chapter 2 gives an overview of the current processes for CO 2 removal and the possibility of promoters. The next chapter contains information about the use of piperazine as a promotor for alkanolamines. The recent discovery to use piperazine as a promotor for hot potasium carbamate systems will be discussed in chapter 4. In the last chapter the use of piperazine is discussed and conclusions are drawn from the gathered information.

7 Chapter 2 Overview of the current processes for CO 2 removal Removal of CO 2 from gases can be split up into two different methods. Firstly, the removal with alkanolamines by which the CO 2 is removed by absorption reactions. Depending on the process requirements, several options for alkanolamine based treating 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 CO 2 with alkaline salts, for example sodium- or potassium carbonate. The major processes are based on aqueous solutions of sodium and potassium compounds. 2.1 Procedures with alkanolamines Bottoms [11] firstly described the absorption of CO 2 by tri-ethanolamine (TEA). His patent was used for early gas-treating plants. Further research showed that other alkanolamines could absorb CO 2 as well. The most important and used alkanolamines, shown in figure 2.1, are: a. monoethanolamine (MEA) b. diethanolamine (DEA) c. methyldiethnolamine (MDEA) HO C C NH 2 HO HO C C C C NH HO HO C C C C CH 3 (a) MEA (b) DEA (c) MDEA Figure 2.1: Structure of the alkanolamines Reaction mechanism of alkanolamines To understand and clarify the absorption mechanism of CO 2 by alkanolamines there has to be discriminated between the primary, secondary, and tertiary amines.

8 Chapter 2. Overview of the current processes for CO 2 removal 8 The mechanism of the primary and secondary alkanolamines MEA and DEA can be represented as the general equations (2.1) and (2.2). The overall forward reaction between CO 2 and primary and secondary alkanolamines usually has been represented as: CO 2 + R 1 R 2 NH R 1 R 2 NCOOH (2.1) R 1 R 2 NCOOH + R 1 R 2 NH R 1 R 2 NCOO + R 1 R 2 NH + 2 (2.2) The first step is bimolecular, second-order, and rate determining, while the second step was supposed to take place instantaneously. However, this scheme is a substantial simplification for the reaction mechanism that actually occurs. Since 1960 a large number of studies on the reaction between CO 2 and alkanolamines in aqueous solutions 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 MEA a general agreement exists on both the reaction order and the value of the kinetic constant. Versteegh and Oyevaar [33] have shown for the alkanolamine DEA that the reaction with CO 2 occurs via the zwitterion type mechanism. For this amine no generally valid, unique reaction order can be represented. For other primary and secondary alkanolamines overall reaction orders varying between two and three were obtained, both for aqueous and non-aqueous solutions. Therefore the mechanism of the reaction between CO 2 and alkanolamines is not as simple and straightforward as suggested by the equations (2.1) + (2.2), even for MEA. Danckwerts [16] reintroduced a reaction mechanism proposed originally by Caplow [12] which described the reaction between CO 2 and alkanolamines via the formation of a zwitterion followed by the removal of a proton by a base, B: CO 2 + R 1 R 2 NH R 1 R 2 N + HCOO (2.3) R 1 R 2 N + HCOO + B R 1 R 2 NCOO + BH + (2.4) Research by Donaldson and Nguyen [17] has shown that for a ph < 11 this reaction can be described with the base catalysis of the CO 2 hydration: CO 2 + R 1 R 2 R 3 N + H 2 O R 1 R 2 R 3 NH + + HCO3 (2.5) Versteeg and Van Swaaij [34] and Benitez-Garcia et al. [5] demonstrated that the absorption of CO 2 in aqueous triethylamine solution is identical to alkanolamines. Therefore the observed reactivity of tertiary alkanolamines towards CO 2 at lower ph-values of the aqueous solution could not be attributed to the formation of monoalkylcarbonate. Furthermore, according to the mechanism proposed by Donaldson and Nguyen [17], no reaction should occur if CO 2 is absorbed into a non-aqueous tertiary amine solution. Versteeg and Van Swaaij have shown that the absorption rate of CO 2 into MDEA-ethanol could be described completely as a physical absorption. It was almost identical to the absorption of N 2 O, corrected for the differences in physical constants, in the same solution. Moreover, the total amount of CO 2 absorbed was nearly the same as the amount which can be physically dissolved in this solution. They were able to ascribe the differences completely to the presence of primary and secondary amine impurities.

9 Chapter 2. Overview of the current processes for CO 2 removal 9 From this result it is easy to conclude that in non-aqueous solutions no reaction, even alkylcarbonate formation, occurs between CO 2 and tertiary amines. This is in good agreement with the proposed reaction mechanism. The reaction of tertiary amines does not occur in non-aqueous solutions Advantages and disadvantages of several alkanolamines In the industry many different alkanolamines are used. To understand why a amine is used in a specific situation the differences need to be shown. Formerly MEA was used in gastreating plants for CO 2 removal. But MEA is rapidly displaced by more efficient systems. However, for low concentrations of H 2 S and CO 2 it is prefered. This is especially for low pressure and maximum removal of H 2 S and CO 2. The advantages are the high alkalinity and easy recovery from contaminated solutions. On the other hand an irreversible product is formed with COS and CS 2, it is more corrosive, high heat of reaction with CO 2 and also a high vapour pressure [22]. DEA is used for the treatment of refinery gases with COS and CS 2. DEA is less reactive with COS and CS 2 than MEA and also a lower vapour pressure is needed. The disadvantages of DEA treatment are that vacuum distillary is necessary and therefore difficult. And when a high content of CO 2 is present, DEA isn t a good choice because of the forming corrosive degradation products [22]. MDEA is another alkanolamine which can be used for the removal of CO 2. The advantages given by Appl et al [1] are low energy requirement, high capacity and high stability, the disadvantage the low rate of reaction with CO 2. The rate of the reaction can be increased by using promoters, without diminishing the MDEA advantages. The different promoters will be discussed in section CO 2 removal with alkaline salts As mentioned in the introduction, CO 2 can also be removed from the gases by the use of alkaline salts. These salts are sodium and potassium carbonate, phosphate, borate, arsenite and phenolate. The major processes are based on aqueous solutions of sodium and potassium compounds. The hot potassium carbonate process is effectively used in many ammonia, hydrogen, ethylene oxide and natural gas plants. To improve CO 2 absorption mass transfer and to inhibit corrosion, proprietary activators and inhibitors are added. These systems are known as activated hot potassium carbonate (AHPC) systems [22]. Which type of activators are used will be discussed in section 2.3. The accepted mechanism of CO 2 absorption into water consists of two parallel mechanisms: 1. Direct formation of HCO 3 CO 2 + OH HCO 3 (fast) (2.6) HCO 3 + OH CO 3 + H 2O (instant) (2.7)

10 Chapter 2. Overview of the current processes for CO 2 removal Reaction of CO 2 with water followed by dissociation of carbonic acid CO 2 + H 2 O H 2 CO 3 (slow) (2.8) H 2 CO 3 + OH HCO 3 + H 2O (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, Savage [30] 2.3 Promoters and/or activators used in CO 2 absorption Because of the low reaction rate of the CO 2 removal by alkanolamines or alkaline salts, promotors or activators are needed to improve the absorption process. The following 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]: CO 2 + 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 homogeneous catalysis [3, 31] In the Hot Potassium carbonate process, of Benfield [13], the alkanolamine DEA is used as the activator of the process, but Bartoo [4] discovered a new organic promotor. This new activator has a better absorption rate than DEA, at lower vapour pressure. Another advantage is the excellent chemical stability. The activator is called ACT-1. The Giammarco Vetrocoke [19] process uses glycine as an activator. This leads to an increase of 18% plantload compared to DEA. In dual-activated (amine + glycine) systems a lower CO 2 vapour pressure, a higher regenerator efficiency and a higher O 2 absorption rate is detected, then when a mono activated solution is used. Addition of arsenic trioxide to aqueous sodium or potassium carbonate solutions results in a marked increase in the rate of absorption and a desorption of carbon dioxide, when it is compared with the conventional carbonate solutions. The reaction rate increases because the rate of hydration of carbon dioxide to carbonic acid is increased. There is also a shift of the ph toward the acid side in the regeneration step, 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 widespread application in the absorption and removal of CO 2 from process gases.

11 Chapter 2. Overview of the current processes for CO 2 removal 11 The success of these solvents is due to the high rate of reaction of the primary or secondary amine with CO 2 combined with the low heat of reaction of the tertiary amine. By adding small amounts of the primary or secondary amine, a high rate of absorption is seen in the absorber, while a low energy of regeneration is required in the stripper. One such blend of amines is piperazine (PZ) activated methyldiethanolamine (MDEA). These solvents have been used successfully for high capacity CO 2 removal in ammonia plants and are patented by BASF [1]. In the next chapter a more detailed description will be given of the use of piperazine.

12 Chapter 3 Piperazine as activator for alkanolamines In this chapter the influence of piperazine (PZ) on the reaction of alkanolamines with CO 2 is described. PZ is most active as promotor when used in combination with MDEA, therefore the most attention is payed to articles concerning this combination. 3.1 Chemical data of piperazine In table 3.1 shows an overview of the chemical data of PZ. Synonyms: Piperazine Anhydrous, Diethylenediamine Molecular Formula: C 4 H 10 N 2 Formula Weight: Registry number: Density: 146 Melting point C Boiling point C Flash point 82 C N Structure N Table 3.1: Chemical data of piperazine PZ may be synthesised by, for example, reacting monoethanolamine with ammonia, or reacting ethylene oxide and NH 3 and cyclising the ethanolamines thereby obtained [1]. The ability of a solvent to remove carbon dioxide is dictated by its equilibrium 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 MDEA has a base catalytic effect on the CO 2 hydrolytic reaction.

13 Chapter 3. Piperazine as activator for alkanolamines 13 Xu et al.[37] was the first who studied the reaction kinetics of PZ with MDEA. In this study it is proposed that the reaction can be considered as rapid pseudo first order with respect to R 3 N. Also the CO 2 absorbed by free PZ can be transferred to MDEA rapidly with itself resumed. As seen in table 3.1 piperazine contains two basic nitrogens and can theoretically react with 2 mol of CO 2. However, the second amine groups ability to bind a second CO 2 can be neglected, which was first reported by Liu et al. [24]. Bishnoi and Rochelle [8] proposed that the second amine is reactive, but the ph of these systems is never low enough to observe di-protonated PZ. It is proposed by Xu et al. [36] that the following reactions occur in a solution of PZ mixed with MDEA: MDEA + H + MDEAH + (3.1) MDEA + CO 2 MDEA CO 2 (3.2) MEA CO 2 + H 2 O MDEAH + + HCO3 (3.3) P Z + CO 2 P Z CO 2 (3.4) P Z CO 2 + H 2 O P ZH + + HCO3 (3.5) P Z CO2 + H 2 O P ZCOO H 3 O + (3.6) P Z + H + P ZH + (3.7) P Z CO 2 + MDEA MDEA CO 2 + P Z (3.8) HCO 3 CO H + (3.9) H 2 O H + + OH (3.10) The mentioned reactions are also shown by Bishnoi and Rochelle [9], but they propose that also the following reactions take place: P ZCOOO + H 2 O + CO 2 P Z(COO ) 2 + H 3 O + (3.11) MDEA + P ZOO + CO 2 P Z(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].

14 Chapter 3. Piperazine as activator for alkanolamines 14 The difference between the two articles is that Bishnoi and Rochelle have equilibrium constants for di-protonated piperazine. To calculate the total conversion of MDEA to 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+ = C MDEA + C MDEAH+ (3.13) Bishnoi and Rochelle have a slightly different PZ balance, because they think diprotonated PZ also exists in the reaction. The difference is viewed in bold in equation (3.14): C P Z = C P Z + C P ZH+ + C P ZCOO + C H + P ZCOO + C P Z(COO ) 2 (3.14) Also the overall CO 2 balance from Bishnoi and Rochelle differs from Xu et al. C HCO 3 + C CO C CO2 + C P ZCOO + C H + P ZCOO + 2C P Z(COO ) 2 = y(c MDEA + C P Z) (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 of PZ with MDEA [8, 9, 36, 37, 38]. Until the article of Bishnoi [9] the overall reaction was proposed as pseudo first-order. Bishnoi proposed that pseudo first-order model is a good assumption for MDEA solutions since the reaction of MDEA is slow enough that no significant depletion of MDEA occurs at the interface. The pseudo first-order curve for PZ/MDEA blends occurs only true at low loading, therefore the kinetics model is changed to use a second-order rate. There is just one article that proposes the use of a second-order model therefore the pseudo first-order model is also shown Kinetics with pseudo first-order model For low loading the pseudo first-oder model is used. This assumes the concentration of 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 dominant reaction of absorption of CO 2 into activated MDEA solution. Simultaneously, the activator PZ may react with CO 2 in liquid film to form an intermediate as: R (NH) 2 + 2CO 2 R (NHCOO) 2 (3.16) This reaction is rapid and runs parallel with reaction (2.1). The hydrolytic reaction of R (NHCOO) 2 also takes place in equilibrium in the liquid phase as: R (NHCOO) 2 + 2H 2 O R (NH + 2 ) + 2HCO 3 (3.17)

15 Chapter 3. Piperazine as activator for alkanolamines 15 If these reactions are taken into consideration the reaction rate has been proposed as [37, 38]: r = (k 2 C am + k p C p )(p CO2 p CO 2 ) (3.18) This expression assumes that the conversion of MDEA and PZ is constant in the liquid film. It is shown by Savage et al. [30] that chemical absorption theory can be applied to chemical desorption in a fast reaction regime. Therefore Xu et al. [37, 38] proposed that the chemical absorption and desorption rate for activated MDEA solutions can be expessed as: N CO2 = H CO2 D CO2 (k 2 C am + k p C p )(p CO2 p CO2 ) (3.19) N CO2 = H CO2 D CO2 (k 2 C am + k p C p )(p CO2 p CO2 ) (3.20) This is only true when the partial pressure of CO 2 is not very high and the free concentration of MDEA is not very low. It is claimed by Bishnoi et al. [8] that the results of Xu et al. [37] are not valid, because of the high partial pressure of CO 2. Therefore the pseudo-first-order assumption breaks down, the reaction occurs 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 the measured enhancement was not significantly different from the enhancement in MDEA solutions, making it impossible to gain any information about the effect of PZ. The results are therefore best described as pseudo-first-order absorption into unpromoted 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 s unreasonable 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.

16 Chapter 3. Piperazine as activator for alkanolamines Kinetics with other models The kinetics model need to be changed, since the model data does not fit the experimental data at higher loading. Bishnoi [9] reviewed the different models to fit the data. The following models are discussed by Bishnoi: Pseudo first-order (E P F O ) Pseudo first-order model, discussed in section Bishnoi used the expression: E P F O ( k Am [Am])D CO2 = kl 2 (3.21) Model with instantanous reactions and small driving force (E GLBL,INST ) The enhancement factor predicted by the simple model of instantaneous reactions and small driving force isn t a good approximation of the data. E GLBL,INST = k l, P ROD kl,co 2 [Am] T H CO2 P CO 2 / α (3.22) Model with only instantaneous reactions for PZ (E P Z,INST ) Good approximation is obtained at high loading if only the PZ reactions are considered instantaneous. This is done by increasing all carbamate and dicarbamate formation rate constants to very large numbers. Combined model off instantanous PZ model and pseudo-first-order (E P Z,INST + E P F O ) The pseudo first-order model and the instantaneous PZ model can also be combined. As seen in Figure 3.3 the results of this model are quite accurate. This model can be expressed as follows: E = 1 1 E P Z,INST + 1 E P F O (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 be found. 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 the effect of piperazine needs to be compared to other promoters.

17 Chapter 3. Piperazine as activator for alkanolamines 17 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 as function of the CO 2 concentration for the primary amines MEA(25%), DGA(25%), KGl(4.0M), the secondary amine DEA(50%), the tertiary amine MDEA(50%), as well 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 MDEApiperazine system can be a promising solvent for recovering CO 2. Figure 3.4: Comparable absorption rates of different aqueous amine systems at 40 C, as function of CO 2 concentration [18]. Later on Bishnoi [9] compared the MDEA-piperazine blend with DEA/MDEA and MEA/MDEA blends. It was seen that the piperazine blends enhances the performance of MDEA by a factor of 50 at low loading. Even at high loading (0.5 mol CO 2 /mol amine), the piperazine blend can be up to a factor of 5 more effective then 50% MDEA. Piperazine is found to be a more effective promoter than MEA or DEA, especially at low loading. The high driving force is not seen to have a major effect on the predicted enhancement factor except at high loading.

18 Chapter 3. Piperazine as activator for alkanolamines 18 The decrease in enhancement is due to the depletion of piperazine and piperazine carbamate 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 is proposed by Seo [32] that the addition of piperazine to an aqueous mixture of 2-Amino-2-methyl-1-propanol (AMP) increases the reaction rate constant remarkably. The apparent rate coefficient increased with increasing piperazine concentration. In Figure 3.6 the apparent reaction rate constants for the reaction of CO 2 into aqueous 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 AMP concentration. It is proposed that the decrease of promoter effect is the result of the decrease 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 reaction of CO 2 with aqueous solutions AMP at 40 C [32] Not mentioned before is the use of piperazine as promoter in combination with MEA. A recent study of Rochelle et al. [27] showed that piperazine can also used as

19 Chapter 3. Piperazine as activator for alkanolamines 19 promoter for MEA systems. An addition of 0.6M PZ in 1.0M MEA increases the rate by a factor 2 to 2.5 at 60 C. The relative effect of piperazine is practically independent of CO 2 loading, except at very high loading. This is because PZCOO is also a reactive species with CO 2. Only at very high loading ( ), are both PZ and PZCOO depleted. From this data it may be concluded that piperazine is also promising promoter for MEA systems.

20 Chapter 4 Piperazine as activator for hot carbonate solutions The hot carbonate process for CO 2 removal from gas streams is of interest because of the high efficiency it exhibts. This process usually operates at temperatures around 100 C. At this temperature chemical absorption and desorption takes place, and it is believed that the liquid side mass transfer rates are significantly enhanced by chemical reactions. The most recent discovery is the use of piperazine as promotor for aqueous potassium carbonate systems. Because of this recent discovery there is only one unpublished article [14] that describes that effects of piperazine on the absorption of carbon 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 shown by serveral investigators [6, 7] that potassium carbonate has a low heat of regeneration, but its rate of reaction is slow compared to amines. 4.1 Absorption and desorption of CO 2 from hot carbonate solutions To understand the influence of piperazine the reaction kinetics without piperazine is described first. The overall reaction can be described as: CO 2 + CO 2 3 2HCO 3 (4.1) In absorption the reaction (4.1) proceeds from left to right, and takes place essentially through the following sequence of elementary steps: CO 2 + OH HCO 3 (4.2) H 2 O H + + OH (4.3) CO H + HCO 3 (4.4) Reaction (4.2) is rate controlling, and the reactions (4.3) and (4.4) are occuring everywhere at equilibrium. Another mechanism proposed by Wall [35] known as the direct reaction of CO 2 with water is not be accounted. Because the contribution to the overall reaction is negligible unless the ph of the liquid solution is very low.

21 Chapter 4. Piperazine as activator for hot carbonate solutions 21 It is proposed by Savage [30] that the kinetic constant can be calculated with the film theory equations. The results of this kinetic constant indicates that, at temperatures as high as 105 C, the rate of the chemical reaction is not large enough for the reaction itself to be regarded as instantaneous. Therefore, even at the highest tempratures of industrial practice the possibility of rate promotion by additives is of significant interest Kinetics with un-promoted carbonate solutions Since reaction (4.2) is the rate controlling step, the rate equation can be described as [30]: r OH = k OH [OH ][CO 2 ] k OH [HCO 3 ] (4.5) Where k OH and k OH are forward an backward rate constants of reaction (4.2). At equilibrium conditions equation (4.5) leads to: k OH [HCO 3 ] = k OH[OH ][CO 2 ] e (4.6) The equilibrium concentration of CO 2 is defined as [CO 2 ] e. The expression for the reverse reaction (4.2) in equation (4.6) has been evaluated by considering conditions at equilibrium, but it is generally true, even when the system is not at equilibrium [15, 23]. Substituting equation (4.6) into (4.5) gives: r OH = (k OH [OH ])([CO 2 ] [CO 2 ] e ) (4.7) Carbonate bicarbonate system is a buffer solution, so the concentration of OH ion in the solution near the surface of liquid is not significantly depleted by the absorbed CO 2. In this case, the carbon dioxide undergoes a pseudo-first order reaction and equation (4.7) may be rewritten as [3, 15]: where k 1 denotes apparent first-order rate constant. r OH = (k 1 ([CO 2 ] [CO 2 ] e ) (4.8) 4.2 Absorption with promoted carbonate solutions Since there is just one article where piperazine is used as promoter for carbon dioxide absorption, most of the kinetics are based on the theory of other amines. When the results are compared there are not many differences so it is believed that piperazine has a comparable reaction mechanism and the same kinetics Kinetics with promoted carbonate solutions When a small amount of amine is added into a carbonate solution, the absorption rate is enhanced greatly according the following reactions: [3] CO 2 + RR NH RR NCOOH (4.9) RR NCOOH + OH HCO 3 + RR NH (4.10)

22 Chapter 4. Piperazine as activator for hot carbonate solutions 22 At higher temperatures, in the range of industrial operating conditions, the rate of reaction (4.10) increases significantly. Therefore the system is better represented by the homogeneous catalysis mechanism [3, 15] and reaction (4.9) is the ratecontrolling step. It is proposed by Rahimpour et al. [25] that the rate equation of carbon dioxide with promoted hot potassium carbonate in liquid phase can be described as pseudo first order: r = (k OH [OH ] + k Am [Am])([CO 2 ] [CO 2 ] e ) = k([co 2 ] [CO 2 ] e ) (4.11) where k is the overall apparent first-order rate constant and is defined as: k = (k OH [OH ] + k Am [Am]) (4.12) When reaction (4.9) is not rate-controlling, but reaction (4.10) is rate-controlling the rate equation can be described as follows [14]: r = k f [CO 2 ][Am] k r 1 + 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, k b [B], is large and the denominator reduces to a value of one. When deprotonation of the intermediate (reaction (4.12)) is ratecontrolling, k b [B], is small so that the denominator is taken into consideration. In addition to chemical reaction, mass transfer becomes an important consideration in 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 the penetration surface renewal theory developed by Danckwerts [15]. The theory that Cullinane [14] uses is known as the eddy diffusivity theory. This theory has the advantage that it is time-independent. When the penetration surface renewal theory is used the mass transfer can be described as: N CO2 = Ek l (C CO2i C CO2e ) (4.14) Where C CO2i is the concentration of carbon dioxide at the interface and C CO2e is the equilibrium concentration of unreacted carbon dioxide in the bulk of liquid when the reverse reaction of carbon dioxide is appreciable. k l is liquid phase mass transfer coefficient and E is the enhancement factor and describes the mass transfer coupled by chemical reactions as[15]: E = 1 + D CO 2 k kl 2 (4.15) Where k is defined by equation (4.12). The rate of absorption which is defined by equation (4.14), can be rewritten in terms of physical solubility of carbon dioxide in solution, H, in the reactive K 2 CO 3 solution as: In the gas phase the mass transfer as: N CO2 = k L HE(P CO2i P CO2e ) (4.16) N CO2 = k gco2 (P CO2 P CO2i ) (4.17)

23 Chapter 4. Piperazine as activator for hot carbonate solutions 23 Where k gco2 is the gas phase mass transfer coefficient of carbon dioxide. The total mass transfer is found by combining equations 4.16 and 4.17 and eliminating the interface partial pressure of carbon dioxide, P CO2i. This gives the following equation: Where K gco2 N CO2 = ( kgco2 k LEH k gco2 + k L EH ) (P CO2 P CO2e ) = K gco2 (P CO2 P CO2e ) (4.18) is overall gas phase mass transfer coefficient of carbon dioxide. The eddy diffusivity theory for carbon dioxide can be described by Cullinane [14]: [ (D CO2 + ɛx 2 ) [CO ] 2] + R CO2 (4.19) a x In this equation, the diffusion of CO 2 is important near the gas-liquid interface. The value of D/ɛ as it occurs in the solution is approximated by the diffusion coefficient of CO 2 [14]. 4.3 Results with piperazine as promotor Rahimpour et al. [25] and Cullinane [14] proposes that the addition of an amine increases the absorption rate dramaticly. When piperazine is added the rate behavior of this solvent approaches that of 5M MEA at both 40 C and 60 C. At a rich loading, both promoted K 2 CO 3 solutions compare favourably with a MDEA/piperazine blend, as mentioned in section 3.4. When the amine amount is increased beyond a specific amount, the promoter effect decreases. Rahimpour et al. [25] proposes that this can be explained due a higher enhancement factor in the liquid phase, which is directly proportional to the overall mass transfer coefficient in the case of liquid-phase controlled mass transfer. By increasing the amine concentration, the gas phase mass transfer is considered the major factor controlling the absorption process so the CO 2 removal is unaffected by increasing the promoter concentration. Cullinane [14] compared the PZ-promoted K 2 CO 3 with other promoters used in K 2 CO 3 solutions diethanolamine (DEA) and an unspecified amine investigated by Satari and Savage [29]. The results are seen in Figure 4.1. For this promoter comparison, CO 2 loading was represented as the conversion of CO 2 3 to HCO 3 and Henry s constant was estimated accounting only for the K 2CO 3 in solution. While each promoter improves the rates over un-promoted K 2 CO 3 to some degree, piperazine at 60 C gives the best improvement. It is proposed that at the temprature of 90 C piperazine is much more favourably then the promoter DEA and the hindered amine. This behaviour can be partially attributed to improved rate behaviour and partially to salting out CO 2 at high temperatures and high ionic strengths [14].

24 Chapter 4. Piperazine as activator for hot carbonate solutions 24 Figure 4.1: Comparison of promoted K 2 CO 3 solutions It is concluded by Cullinane [14] that the heat of absorption of CO 2 increases with the addition of PZ to aqueous potassium carbonate. With a comparable loading and piperazine concentration, more potassium carbonate serves to decrease the heat of absorption only slightly, indicating that the amine is largely responsible for the reaction with CO 2. A decrease in loading results in a marked increase in the heat of absorption, most likely due to a difference in heats of absorption of piperazine and of piperazine dicarbamate. The results also suggest that promoted potassium carbamate solutions would possess a lower heat of absorption then comparable amine systems. An overview of the results is displayed in figure 4.2 [14]. Figure 4.2: CO 2 heat of absorption in K 2 CO 3 /PZ

25 Chapter 4. Piperazine as activator for hot carbonate solutions 25 Proton NMR done by Cullinane suggests that piperazine carbamate is the dominant species 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 concluded that loading has a significant effect on absorption rates.

26 Chapter 5 Conclusions The current processes for CO 2 removal are used on a large scale. Most processes are based on well known data and therefore optimising the processes is not really an 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 first who researched the influence of piperazine on the CO 2 removal. It was shown that piperazine accelrates the absorption considerably. The effect of piperazine on CO 2 removal is researched in depth since The first published 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 groups have published it can be concluded that the MDEA-piperazine system is a promosing solvent for CO 2 removal. It is concluded that a solvent of piperazine/mdea (5 wt. %/45 wt. %) provides almost two orders of magnitude and more enhancement then 50 wt. % MDEA at low loading and one order of magnitude enhancement at moderate loading. The use of piperazine as promotor for the CO 2 absorption in aqueous potassium carbonate is recently found to be effective [14, 25]. Piperazine increases the absorption rate of CO 2 substantially. Current studies reveal that, coupled with the low heat of absorption associated with aqueous K 2 CO 3, the piperazine/k 2 CO 3 system could potentially reduce energy costs associated with CO 2 removal. More studies are needed for this solvent over a broader range of industrially significant conditions. Since most articles are from just two research groups (University of Texas and East China University) it is necessary that more research is done by different research groups. Also because the research group from the East China University is not always that accurate with their results [26]. The overall conclusion from the data presented in this thesis is that piperazine is an effective promotor in combination with MDEA and aqueous potassium carbonate. For usage on an industrial level further research is however needed.

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