Precipitation of Piperazine in Aqueous Piperazine Solutions with and

Aug 28, 2012 - The crystallization of piperazine in water as well as in systems loaded with CO2 has been studied for piperazine concentrations of 30â€...
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Precipitation of Piperazine in Aqueous Piperazine Solutions with and without CO2 Loadings Xiaoguang Ma,† Inna Kim,‡ Ralf Beck,† Hanna Knuutila,‡ and Jens-Petter Andreassen*,† †

Department of Chemical Engineering, NTNU, N-7491 Trondheim, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway

‡

ABSTRACT: The crystallization of piperazine in water as well as in systems loaded with CO2 has been studied for piperazine concentrations of 30−70 wt %, representing conditions relevant for CO2 capture. The use of a LabMax reactor system equipped with probes for in situ focused beam reflectance measurement (FBRM) and particle vision measurement (PVM) made it possible to determine solid−liquid transitions, crystal habit, and chord length distributions in these highly concentrated systems without disturbing the solid−liquid−gas equilibrium during crystallization. As shown by powder X-ray diffraction analysis, three phases including piperazine hemihydrate, piperazine hexahydrate, and anhydrous crystals were precipitated from the aqueous piperazine solutions at different concentrations and temperatures, as also supported by findings from FBRM and PVM. It was found that the metastable zone widths of the piperazine−H2O system were substantial even at the lower cooling rates, which could allow for a higher tolerance with respect to cooling prior to a new carbon dioxide absorption cycle. However, the eutectic composition exhibits a smaller metastable zone width than the other concentrations, which is believed to be caused by the precursor needleshaped crystals, assisting the precipitation of the final product.

1. INTRODUCTION

In the piperazine−H2O−CO2 system, the following reactions may take place:3

Amine-based absorption/stripping systems utilized for removal of CO2 from coal-fired power plants have been widely studied.1 Except for liquid based processes, such as established monoethanolamine (MEA) absorbents, precipitating systems used for CO2 capture have attracted increasing attention in recent years. In such postcombustion CO2-capture processes, absorption and stripping are performed in columns by performing a temperature swing. Carbon dioxide is captured in the absorber at a temperature of 30−50 °C (MEA). The “rich” solvent is then pumped to a stripper, where CO2 is desorbed at the temperature of around 120 °C, thereby regenerating the solvent. The “lean” solvent is sent back to the absorber for a new cycle of CO2 capture. In systems with the potential for precipitation, like K 2 CO 3 /KHCO 3 and (NH4)2CO3/NH4HCO3 (the chilled ammonia process), the selective removal of reaction precipitates (bicarbonates) from the reaction mixture will essentially shift the equilibrium toward the product side, thereby increasing the CO2 absorption capacity. An additional benefit of such processes is offered by the fact that only concentrated slurry needs to be sent to the stripper, thereby reducing both the recycling load and sensible heat requirements. Another promising candidate for CO2 capture is the cyclic ethyleneamine piperazine (Pz) which comprises two secondary amine groups resulting in high reactivity with CO2. It has been shown to be an efficient promoter to enhance the CO2 mass transfer rate in systems such as MDEA/Pz2 and K2CO3/Pz.3,4 Due to the relatively low solubility of Pz, the concentration is normally between 0.5 and 2.5 m to avoid precipitation of piperazine-based compounds.3 In the study of Freeman et al.,5 however, concentrated aqueous piperazine (more than 8 m) was shown to be a promising solvent for CO2 capture by itself. © 2012 American Chemical Society

CO2 (g) ↔ CO2 (aq)

(1)

CO2 (aq) + 2H 2O ↔ HCO3− + H3O+

(2)

HCO3− + H 2O ↔ CO32 − + H3O+

(3)

2H 2O ↔ H3O+ + OH−

(4)

PZH+ + H 2O ↔ PZ + H3O+

(5)

PZ + CO2 + H 2O ↔ PZCOO− + H3O+

(6)

H 2O + H+PZCOO− ↔ H3O+ + PZCOO−

(7)

PZCOO− + CO2 + H 2O ↔ −OOCPZCOO− + H3O+ (8)

Equations 5 to 8 show how piperazine reacts into hydrogenated piperazine and carbamates during the CO2 absorption process. Since the resulting carbamates exhibit higher solubility than piperazine itself, this system behaves differently from the carbonate systems. Precipitation will not happen as a result of CO2 absorption, but rather in the lean solvent, if the concentration of piperazine is sufficiently high. During the desorption of carbon dioxide, the concentration of piperazine will increase as CO 2 is stripped off, and crystallization of piperazine will eventually occur if the concentration of piperazine is sufficiently high and also at Received: Revised: Accepted: Published: 12126

April 27, 2012 June 21, 2012 August 27, 2012 August 28, 2012 dx.doi.org/10.1021/ie301101q | Ind. Eng. Chem. Res. 2012, 51, 12126−12134

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reflectance measurements (FBRM) and particle vision microscope (PVM). The iControl LabMax (Mettler Toledo) software was used to control the experimental parameters such as cooling/heating rates and stirring speed and to record the experimental data. To study the metastable zone, the “Crystallization mode” was used during cooling by setting a constant temperature difference (2, 5, 10, and 15 °C, respectively) between the jacket (Tj) and the reactor interior (Tr) while the heating procedure was achieved by heating the jacket at a constant heating rate of 3 °C/min. The temperature at completion of dissolution during heating was recorded as the solubility of piperazine at a certain concentration, as described previously.17 The deviation of the onset of crystallization (by cooling) from the solubility temperature was determined as the metastable zone width. Piperazine solutions with the concentrations of 35.0, 41.0, 44.3, 60.0, and 65.0 wt % were studied, as well as solutions loaded with different amounts of CO2 into a starting concentration of 70 wt % piperazine. The CO2 was added from a CO2 cylinder with logged on temperature (Pt-100, uncertainty ±0.1 °C) and pressure (Keller Leo 3000 pressure transmitter, 0−30 bar range, uncertainty ±0.1% FS). The amount of CO2 loaded into the solutions was calculated by the change in pressure and temperature in the feed cylinder using the Peng−Robinson equation. FBRM spectra and temperature change were used to record the start of crystallization and complete dissolution. The change of chord length distributions with time and PVM pictures were used for monitoring the size and morphology of crystals. 2.3. XRD Analysis. Suspension samples were collected from the solutions after sufficient yield was obtained and filtered to isolate the crystalline product. For many of the applied concentrations, the contents will be completely solidified given enough time for crystallization, even at quite high temperatures. In order not to destroy the LabMax-equipment, some additional experiments (with the concentrations of 30.0, 44.5, 55.0, 60.0, 65.0, and 85.0 wt %) were performed in a simpler 1-L batch reactor stirred with a magnetic bar, allowing for lower final temperatures, to get more information about solids formed during the cooling crystallization process. The resulting crystals were ground for powder X-ray diffraction analysis.

conditions where the solutions are still partly loaded with CO2. One critical part of the process can be the heat exchanger where the lean solution returning from the reboiler is cooled down to the appropriate absorption temperature. Whether precipitation can be allowed in the process is a matter of process design and the level of piperazine concentrations required for an efficient absorption process. An understanding of the precipitation behavior, involving the solubility and metastable zone width of aqueous piperazine, is important to prevent equipment failure and solid−liquid separation problems. The solubility of piperazine in H2O as well as the resulting solid phases was studied by Rosso and Carbonnel.6 The use of differential scanning calorimetry (DSC) proved the existence of the following solid phases: hexahydrate (Pz·6H2O), hemihydrate (Pz·1/2H2O), and anhydrous piperazine. In recent years, the solid−liquid equilibrium of this system has also been studied by Dow Chemical,7 Bishnoi,8 Hilliard,3 Freeman,5 and Muhammad.9 However, none of these researchers mentioned the existence of piperazine hemihydrate. More recently, Fosbøl et al.10 measured the solubility of piperazine in water at piperazine concentrations ranging from 5.21 to 90.7 wt % using freezing-point depression (FPD) equipment. Piperazine hemihydrate was mentioned as the primary solid products at concentrations from ∼61 to ∼71 wt %. The tolerance with respect to subcooling during the process is related to the metastable zone width (MZW); the temperature difference between the solubility concentration and the supersaturation limit at which spontaneous nucleation occurs for a given cooling rate. The corresponding degree of supersaturation will affect both nucleation and crystal growth11 to give differences in size, shape, and polymorphism of the resulting solid phase, which influence the solid−liquid separation qualities of the resulting slurry in the event of precipitation during CO2 absorption with piperazine. A variety of measurement methods have been applied to detect the onset of crystallization to obtain the metastable zone width, such as visualization,12 electrozone sensing,13 and ultrasonic measuring technology14 as well as probes for in situ focused beam reflectance measurement (FBRM) and particle vision measurement (PVM).15,16 In this work, in order to maintain the integrity of the gas−liquid−solid equilibria, the metastable zone width and the crystallization and dissolution behavior, as well as the size and shape of the crystalline piperazine, were studied using FBRM and PVM. Solid phases were also determined by powder X-ray diffraction (XRD). Understanding of the precipitation behavior of piperazine is of high importance in order to prevent equipment failure and solid−liquid separation problems. In this work, we therefore focus on the determination of metastable zone widths and solubility data using in situ techniques to determine the crystallization characteristics of highly concentrated piperazine solution.

3. RESULTS AND DISCUSSION 3.1. Solubility Data and Metastable Zone Width of the Pz−H2O System. When piperazine is used as a solvent for CO2 capture, precipitation will most likely happen after solvent regeneration. For this reason, the binary Pz−H2O system will be of special interest with respect to crystallization and solidification of the contents in equipment parts leading back to the absorber unit. The regeneration will probably not necessarily be complete during operation, and the effect of different loadings is presented in section 3.4. The transition temperatures of piperazine in water, for both dissolution and crystallization, are presented in Figure 1. The data for dissolution temperatures by this approach agree well with solubility data in the literature due to the fast dissolution kinetics of piperazine.17 As for the solubility curve established by Fosbøl,10 it is clear that the dissolution temperature increases with increasing concentration from ∼17 to 44.3 wt % (representing the composition of piperazine hexahydrate). After that, a eutectic point appears at ∼61 wt %, and the solubility temperature increases substantially at higher concen-

2. MATERIALS AND METHODS 2.1. Materials. Piperazine (CAS 110-85-0, 99%, Acros organics) was used as received without further purification. Aqueous solutions were prepared by weighing piperazine and distilled deionized water. CO2 gas (grade 5.0) was used in the loaded experiments. 2.2. Experimental Procedure. Piperazine solutions were prepared by dissolving anhydrous piperazine in water and charged into a 1-L stirred glass reactor in a LabMax (Mettler Toledo) system equipped with probes for in situ focused beam 12127

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trations. When the concentration reaches 75 wt %, a peritectic reaction will occur and piperazine hemihydrate (Pz·1/2H2O) will form.10 The crystallization temperature however is dependent on the rate of generation of the supersaturation and the nucleation kinetics. It is thus expected to decrease with increasing cooling rate at a given concentration, as represented by the metastable zone width. Barrett and Glennon15 determined the metastable zone width of aqueous potash alum solutions with the cooling rate ranging from 0.2 to 0.7 K/min. The resulting MZW increases from 7 to 17 °C for a saturation temperature of 25 °C. The effect of cooling rate was also shown by Titiz-Sargut and Ulrich18 for unseeded potassium nitrate solutions measured by means of an ultrasonic technique using a protected sensor; the MZW increases from around 3 to 6 °C at cooling rates of 9.58−29.8 K/h. The cooling rates in this study were obtained from a criterion of a certain temperature difference, ΔT, between the reactor content and the jacket (The LabMax crystallization mode). This resulted in slightly varying cooling rates as a function of the piperazine concentration due to the different heat capacities of the solutions. Cooling rates, temperature difference ΔT, transitions temperatures, and MZW, are given in Table 1. The transition temperature data are average values based on a minimum of three parallel measurements. The MZW values show that the cooling rates have no systematic effect on the transition temperature of crystallization, like what is usually found in the literature. The subcooling is substantial even at the lower cooling rates. This could be a benefit for the industrial application of piperazine in CO2 capture systems. The magnitude of the metastable zone width provides a certain tolerance to avoid occurrence of crystallization in columns or heat exchangers when a proper concentration is chosen. It should be noted that the metastable zone width at the eutectic point is much smaller compared with the other concentrations, as explained in connection to Figure 5. Caution should be shown when analyzing MZW information in impure industrial solutions. Previous studies have shown that the nucleation kinetics could be impacted by chemical impurities in the way of enlargement or suppression of the MZW, depending on both the ion species and their concentrations.11,14,19,20 Some ions have relatively weak effect on the metastable zone width.19

Figure 1. Crystallization and dissolution temperatures of piperazine at different concentrations.

Table 1. Crystallization and Dissolution Temperatures of Piperazine Solutions with Different Concentrations at Different Cooling Rates transition temperature, °C concentration, wt % 30

41

44.3

60

65

ΔT, °C

cooling rate °C/min

2 5 10 15 2 5 10 15 2 5 10 15 2 5 10 15 2 10 15

0.23 0.40 0.82 1.16 0.27 0.49 0.85 1.22 0.32 0.56 0.96 1.35 0.33 0.59 0.99 1.40 0.37 1.05 1.43

cooling

heating

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

38.3 ± 0.3

24.1 25.1 26.0 23.4 27.6 27.5 27.1 31.3 32.2 31.8 32.0 30.5 32.6 32.0 31.6 31.6 35.1 35.7 36.7

0.4 1.0 3.1 0.1 0.2 0.3 0.4 1.0 0.9 0.1 0.6 0.6 0.2 0.3 0.2 0.7 0.6 0.9 0.2

42.8 ± 0.1

43.0 ± 0.0

32.9 ± 0.1

46.0 ± 1.2

metastable zone width, °C 14.2 13.2 12.3 14.9 15.2 15.3 15.7 11.5 10.8 11.2 11.0 12.5 0.3 0.9 1.3 1.3 10.9 10.3 9.3

Figure 2. Reference XRD patterns of anhydrous piperazine, hemihydrate, and hexahydrate. 12128

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Figure 3. XRD patterns of crystals obtained from (a) 44.3 wt % and 35 wt %; (b) 85 wt %; and (c) 55 wt % and 60 wt % piperazine solutions as well as the corresponding reference patterns.

3.2. Powder X-ray Diffraction Analysis. Piperazine crystallizes as piperazine hexahydrate (Pz·6H2O), hemihydrate (Pz·1/2H2O), or anhydrous piperazine, depending on the solution concentration. The powder XRD patterns for these solids are shown in Figure 2. Since the reference pattern of anhydrous piperazine cannot be found in the XRD database, the commercial piperazine supplied by Arcos (anhydrous) was analyzed as a reference for anhydrous piperazine. When it comes to the hemihydrate, the reference XRD pattern is absent as well. According to the phase diagram in Figure 1, the primary solid products precipitated from the solutions ranging from 61.5 to 75 wt % should be hemihydrate. Therefore, the corresponding XRD pattern of primary precipitates as a result

of the 65 wt % solution was assumed as the reference for hemihydrate. Powder XRD patterns obtained from aqueous Pz solutions with different concentrations in this work are shown in Figure 3. It is evident from Figure 3a that crystals acquired from 30 and 44.3 wt % solutions are piperazine hexahydrate. XRD patterns of primary products from highly concentrated solutions (85 wt %) in Figure 3b indicate the formation of the anhydrous piperazine. Later on, the solid composition should change according to the phase diagram; however, it was not possible to analyze the final products in this case due to inhomogeneous solidification of the reactor contents. Figure 3c shows the XRD spectra of precipitates from 55 wt % and 60 wt % solutions, indicating the formation of mixture of hexahydrate 12129

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metastable zone width. After that, crystals start to freeze accompanied by the release of latent heat of crystallization keeping the system at a constant temperature of 43.0 °C, representing the thermodynamic solubility temperature of this solid phase. This temperature hence represents the highest temperature at which complete solidification might occur, which is of importance considering the safety and operational feasibility of the industrial application. In the experiments, the heating process was started before the completion of solidification to avoid destroying the equipment. At point D, the crystals dissolved completely and the temperature started to rise again. The piperazine hexahydrate crystals easily adhere to the probe of PVM, and the images are of too low quality to show the morphology of Pz·6H2O. It should be noted that, at this composition, the content in the reactor will solidify totally when the temperature is lower than 32 °C, which could be quite critical for industrial application. When the solution is circulated in absorbing and stripping columns, the low temperature (lower than 32 °C) could lead to the complete freezing in heat exchangers and water coolers, resulting in the failure of the whole process. Figure 5 illustrates the cooling and heating curve for the Pz solution of the eutectic composition (60 wt %). The temperature of the liquid phase decreases along curve EF until precipitation happens at point F, where the PVM images show that the crystals are of a needle-like shape. After a slight increase in the solution temperature due to crystallization, the Tr curve decreases in the next few minutes until point G where the needle-like crystals start to transform to flat rhombohedral shaped crystals and then stabilizes at the eutectic temperature of 32.9 °C. Due to the short lifetime of the needles and the inherent transformation, it was not possible to detect this possible polymorph by XRD analysis. The transformation process was well documented by the combination of PVM and FBRM.

Figure 4. Cooling/heating curves for a 44.3 wt % piperazine solution. Tj and Tr are the jacket and reactor temperatures, respectively.

and hemihydrate, which fits well with the phase diagram of the Pz−H2O system present in Figure 1 as long as the temperature is kept at or below the eutectic point. The fact that the peaks corresponding to the hemihydrate phase from the 60 wt % solution are higher than those from 55 wt % solution means more hemihydrates are generated in the 60 wt % solution. 3.3. Cooling/Heating Curves for Aqueous Piperazine Solutions at Varied Concentrations. The in situ crystallization behavior of selected concentrations is shown in the following: the composition (44.3 wt %) corresponding to that of Pz·6H2O and the eutectic point composition (60 wt %) as well as a hypereutectic (65 wt %) and a hypoeutectic (30 wt %) composition. The cooling crystallization curve for a concentration of 44.3 wt % solution is shown in Figure 4. The crystallization results in the phase change of Pz·6H2O from liquid to solid, which eventually leads to complete solidification of the mixture. A smooth cooling curve AB can be observed before the crystallization occurred at point B representing the

Figure 5. Cooling/heating curves and PVM images for a 60 wt % piperazine solution. The PVM pictures show the transition of crystals from needle (F) to rhombohedra (H). The scale bar in PVM pictures is 200 μm. 12130

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Figure 6. Combination of cooling/heating curve and corresponding FBRM spectra of the 60 wt % piperazine solution as well as FBRM trends of crystals with varied morphologies (shown as the inset curves). The FBRM trends of needle and rhombohedral crystals were recorded at the time of 02:03:12 and 02:55:22, respectively.

This lower tolerance with respect to precipitation should be considered when operating this process industrially. Figure 7 shows the cooling/heating curve of a 30 wt % piperazine solution. The first change in point L corresponds to the onset of crystallization, and the system temperature rises up to ∼36 °C at point M. It should be noted that, instead of the constant-temperature plateau as presented in Figure 4, there is a slight decrease of temperature along curve MN, due to the excess of water compared to the eutectic composition. When the heating process begins, the system temperature starts to rise at a slow heating rate until point O, corresponding to the completion of dissolution. After that, the homogeneous liquid is heated up quickly. The cooling and heating curve of a 65 wt % solution is shown in Figure 8a. It agrees well with the freezing behavior of the typical hypereutectic solution.11 The temperature of the uniform liquid phase drops along the line QR until the crystallization occurs at point R where hemihydrous piperazine crystals are generated. In the following stage, the solution temperature keeps falling (curve ST) as more hemihydrous piperazine is produced, resulting in a reduced concentration in the remaining solution until the eutectic composition is reached. Then, eutectic solidification occurs at a constant temperature along the line TU. The PVM picture in Figure 8b presents the rhombohedral shape of hemihydrous piperazine generated at the beginning of crystallization (curve ST). Crystals obtained after the eutectic crystallization are shown in Figure 8c. Crystals obtained after the eutectic crystallization are shown in Figure 8c. Due to the existence of hemihydrates prior to the crystallization at the eutectic composition, the morphology of crystals in Figure 8c looks different from that of Figure 5. At this stage, the solids yield is very high and the mixture will fully solidify if kept for sufficient time at this temperature. During the heating process, point U represents the complete dissolution of the eutectic mixture and point V is the complete

Figure 7. Cooling/heating curves of a 30 wt % piperazine solution. Tj and Tr are the jacket and reactor temperatures, respectively.

The FBRM spectra for the transformation that occurred from point F to G are shown in Figure 6. It is clear that the needlelike crystals exhibit higher chord lengths than the rhombohedral ones (in-set). Therefore, the decrease of the number of counts from F to G can be explained by the dissolution of needles, while the following increase of chord length counts is caused by the formation of rhombohedral crystals. The higher slope of the time dependent chord number graph in the 0−23 μm interval compared with the 30−85 μm interval from point G to H is due to ongoing nucleation of the eutectic composition, in accordance with the PVM picture in Figure 5. As mentioned previously, the MZW values of piperazine solution with eutectic composition are much smaller in comparison with those at other concentrations. This is believed to be caused by the generation of the precursor needle-like crystal. When the concentration is near the eutectic point, the needle-like crystals form easily even at quite small degrees of undercooling and assist the nucleation of the more stable phase. 12131

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Figure 8. (a) Combination of cooling/heating curve and FBRM spectra of a 65 wt % piperazine solution; (b, c) PVM images of primary crystals and final products. The scale bar in PVM pictures is 200 μm.

Figure 9. Effect of CO2 loadings on crystallization temperatures in the 70 wt % piperazine solution. The solubility data (dissolution temperatures) were measured in a previous study.17

dissolution of the hemihydrous compounds, corresponding to the absence of any measured chord lengths. It should be mentioned that, when the parallel experiments were conducted under the same cooling condition, amorphous-like precipitates could form sometimes. However, the XRD result of the amorphous-like solids shows a crystalline pattern which could

be caused by the quick transformation to crystalline phase during sampling. 3.4. Effect of CO2 on the Crystallization Behavior of the Pz−H2O System. The transition temperatures and MZW values of a 70 wt % solution with varying loadings are shown in Figure 9 and Table 2. The loading of CO2 causes an increase in solubility of the solutions as previously shown by Kim et al.17 12132

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0.08 and 0.16 mol-CO2/mol-Pz were loaded, the needle-like crystals arose first and then transformed to crystal similar to what was found in the unloaded system at a concentration of 60 wt %. As mentioned earlier, a highly concentrated piperazine solution (more than 8 m) is a promising candidate for CO2 capture due to the high CO2 capacity and mass transfer rate.5 Such high concentrations may lead to precipitation of piperazine hexahydrate, hemihydrate, or anhydrous piperazine, depending on the absorbent concentration and degree of regeneration of the solvent in the stripper. The stripping of CO2 can be controlled to prevent occurrence of precipitation during the cooling of the “lean” solvent as it is transported back to the absorber.

Table 2. Crystallization and Dissolution Temperatures of 70 wt % Piperazine Solutions with Different CO2 Loadings temperature, °C loading, mol-CO2/ mol-Pz 0 0.08 0.16 0.28 0.36

cooling 42.9 53.9 52.4 30.0 14.6

± ± ± ± ±

0.5 0.3 0.1 0.9 0.5

heating

metastable zone width, °C

± ± ± ± ±

12.9 1.9 5.6 12.0 16.1

55.8 55.8 58.0 42.0 30.7

0.2 0.6 0.2 0.7 0.4

Soluble hydrogenated piperazine and carbamates forming according to eqs 6−8 will be generated during the CO2 absorption process, resulting in a reduction of the piperazine concentration and a corresponding reduction in the solubility temperature. The crystallization temperature of the solutions should move from the value at 70 wt % in the phase diagram of Figure 1 toward the left as more CO2 is loaded into the system. Figure 9 shows that the solubility behavior of the loaded system is behaving differently as a result of the presence of carbamates and other ions. The metastable zone widths for the solutions with CO2 loading of 0.08 and 0.16 mol-CO2/mol-Pz are smaller than those of solutions with the loading of 0, 0.28, and 0.36, corresponding to the effects found around the eutectic composition in the unloaded system. It seems reasonable that a certain CO2 loading will change the solution from the hypereutectic composition (70 wt %) toward the eutectic composition. This is supported by the PVM images in Figure 10. The decrease of metastable zone width at the CO2 loading of 0.08 and 0.16 can be explained by the formation of needlelike crystals at the beginning of the crystallization process. After

4. CONCLUSION The solid phases crystallized from solutions with different concentrations of piperazine in water were determined by powder XRD. The products as a result of the solutions with concentrations lower than 44.3 wt % were found to be Pz·6H2O. Crystallization experiments performed between the hexahydrate composition (44.3 wt %) and 65 wt % resulted in a mixture of piperazine hexahydrate and hemihydrate, while primary products from solutions over 75 wt % were anhydrous piperazine, consistent with the phase diagram. A eutectic point was found at a temperature of 32.9 °C for an initial concentration of 60 wt % piperazine. Due to the regeneration of piperazine when CO2 is stripped from the system, piperazine might precipitate when the lean solution is cooled prior to a new absorption cycle. It was found that the metastable zone widths of the piperazine−H2O system were substantial even at lower cooling rates. However, the eutectic composition exhibits a smaller MZW than the other

Figure 10. PVM images of crystals obtained from a 70 wt % piperazine solution with different CO2 loadings: (1) no loading; (2) loading 1, (b) within 1 min after crystallization start, (c) 10 min after crystallization start; (3) loading 2, (d) within 1 min after crystallization start, (e) 10 min after crystallization start. The scale bar in the pictures is 200 μm. 12133

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(10) Fosbøl, P. L.; Neerup, R.; Arshad, M. W.; Tecle, Z.; Thomsen, K. Aqueous solubility of piperazine and 2-amino-2-methyl-1-propanol plus their mixtures using an improved freezing-point depression method. J. Chem. Eng. Data 2011, 56, 5088−5093. (11) Mullin, J. W. Crystallization, 4 ed.; Elsevier Ltd, 2004. (12) Nývlt, J.; Rychlý, R.; Gottfried, J.; Wurzelová, J. Metastable zone-width of some aqueous solutions. J. Cryst. Growth 1970, 6, 151− 162. (13) Mullin, J. W.; Jancic, S. J. Interpretation of metastable zone widths. Trans. Inst. Chem. Eng. 1979, 57, 188−193. (14) Titiz-Sargut, S.; Ulrich, J. Influence of additives on the width of the metastable zone. Cryst. Growth Des. 2002, 2, 371−374. (15) Barrett, P.; Glennon, B. Characterizing the metastable zone width and solubility curve using lasentec FBRM and PVM. Chem. Eng. Res. Des. 2002, 80, 799−805. (16) O’Grady, D.; Barrett, M.; Casey, E.; Glennon, B. The effect of mixing on the metastable zone width and nucleation kinetics in the anti-solvent crystallization of benzoic acid. Chem. Eng. Res. Des. 2007, 85, 945−952. (17) Kim, I.; Ma, X. G.; Andreassen, J. P. Study of the solid-liquid solubility in the piperazine-H2O-CO2 system using FBRM and PVM. Energy Procedia 2012, in press. (18) Titiz-Sargut, S.; Ulrich, J. Application of a protected ultrasound sensor for the determination of the width of the metastable zone. Chem. Eng. Process.: Process Intensification 2003, 42, 841−846. (19) Dang, L.; Wang, Z.; Liu, P. Measurement of the metastable zone width of phosphoric acid hemihydrate in the presence of impurity ions. J. Chem. Eng. Data 2007, 52, 1545−1547. (20) Myerson, A. S.; Jang, S. M. A comparison of binding energy and metastable zone width for adipic acid with various additives. J. Cryst. Growth 1995, 156, 459−466.

concentrations, which is believed to be caused by precursor needle-like crystals. The crystallization at different piperazine concentration (and different loadings) may in some cases lead to complete solidification. This is the case for the hexahydrate composition at a relatively high temperature of 43.0 °C and for higher concentrations, at the eutectic temperature of 32.9 °C. This can be of high importance for the safe operation of CO2capture plants in case of unintended shut-downs where the process equipment is cooled below these respective temperatures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Aker Solutions, ConocoPhilips, Det Norske Veritas, Gassco, Hydro, Shell, Statkraft, Statoil, TOTAL, GDF SUEZ, and the Research Council of Norway (193816/S60).

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ABBREVIATIONS MZW = metastable zone width Pz = piperazine Pz·6H2O = piperazine hexahydrate Tr = reactor temperature Tj = jacket temperature FBRM = focused beam reflectance measurement PVM = particle vision measurement m = molality

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REFERENCES

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dx.doi.org/10.1021/ie301101q | Ind. Eng. Chem. Res. 2012, 51, 12126−12134