Low Transition Temperature Mixtures as Innovative and Sustainable

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Article

Low Transition Temperature Mixtures as Innovative and Sustainable CO Capture Solvents 2

Lawien F Zubeir, Mark H.M. Lacroix, and Maaike C. Kroon J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5089004 • Publication Date (Web): 11 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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The Journal of Physical Chemistry

Low Transition Temperature Mixtures as Innovative and Sustainable CO2 Capture Solvents

Lawien F. Zubeir, Mark H.M. Lacroix, Maaike C. Kroon*

Separation Technology Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands.

*

Corresponding author. Phone: +31-40-2475289; Fax: +31-2463966; E-mail: [email protected]

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Abstract The potential of three newly discovered low transition temperature mixtures (LTTMs) is explored as sustainable substituents for the traditional carbon dioxide (CO2) absorbents. LTTMs are mixtures of two solid compounds, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which form liquids upon mixing with melting points far below that of the individual compounds. In this work the HBD is lactic acid and the HBAs are tetramethylammonium chloride, tetraethylammonium chloride and tetrabutylammonium chloride. These compounds were found to form LTTMs for the first time at molar ratios of HBD:HBA = 2:1. First, the LTTMs were characterized by determining the thermal operating window (e.g., decomposition temperature and glass transition temperature) and the physical properties (e.g., density and viscosity). Thereafter, the phase behavior of CO2 with the LTTMs has been measured using a gravimetric magnetic suspension balance operating in the static mode at 308 and 318 K and pressures up to 2 MPa. The CO2 solubility increased with increasing chain length, increasing pressure and decreasing temperature. The Peng-Robinson equation of state was applied to correlate the phase equilibria. From the solubility data, thermodynamic parameters were determined (e.g., Henry’s law coefficient and enthalpy of absorption). The heat of absorption was found to be similar to that in conventional physical solvents (-12.58 to -14.57 kJ.mol-1). Furthermore, the kinetics in terms of diffusion coefficient of CO2 in all LTTMs were determined (10-11 - 10-10 m2.s-1). Even though the CO2 solubilities in the studied LTTMs were found to be slightly lower than those in thoroughly studied conventional physical solvents, LTTMs are a promising new class of absorbents due to their low cost, their environmentally friendly character and their easy tunability, allowing further optimization for carbon capture.


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Keywords: Low transition temperature mixtures (LTTMs); Deep eutectic solvents (DESs); Physicochemical properties; CO2 solubility; CO2 diffusivity.

1

Introduction

Carbon dioxide (CO2) capture from industrial large point sources has become increasingly important since CO2 is a major anthropogenic (produced by human activities) greenhouse gas contributing to global climate change. There are numerous processes to recover CO2 either from the fuel (pre-combustion) or the exhaust gases (post-combustion), such as absorption, adsorption, membranes or a hybrid system combining two technologies (e.g. absorption and membranes). Currently, absorption of CO2 by amine-based solutions is the most feasible technology for post-combustion CO2 capture at the industrial scales.1 Despite their good absorption capacity, the amount of energy involved in the regeneration process is huge. Moreover, amine-based solutions are toxic. Due to their susceptibility to thermal and oxidative degradation2,3 and evaporative losses, amine-based processes lead to significant costs associated with solvent make-up4 and can cause severe damage to the environment and the human health.5 Furthermore, physical solvents can be employed as sorbents. In general, those are most feasible at higher CO2 concentrations than the CO2 partial pressures in most exhaust gas streams. Therefore, they are more suitable for the pre-combustion CO2 process. Efforts to use water as solvent were not successful, since the solubilities of CO2 in water are too low for a commercially deployable process. The Rectisol process, based on the physical solvent methanol, was the first commercial process that has been used for syngas applications where CO2 concentrations were reduced to ppm level.6 The disadvantages of this process are the low operating temperature (200 K) and the complexity. Since 1960 new physical solvents were introduced, among others, Fluor solvent (propylene carbonate), Selexol (dimethylethers of polyethylene glycol) and Purisol (n-methyl-23 ACS Paragon Plus Environment


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pyrrolidone (NMP)). Unlike chemical absorption with much higher heats of absorption, physical solvent processes do not require a regenerator driven by steam or other heat sources. Instead, an atmospheric flash vessel is sufficient to regenerate the solvent. Since physical absorbents reach the highest absorption capacity at the lowest temperature, refrigeration of the solvent may introduce additional costs. In the quest of finding harmless solvents a number of approaches have been proposed to develop ‘green’ solvents suitable for carbon capture7: (i) use of biocompatible solvents, (ii) use of so-called ‘bio-solvents’ produced from renewable resources, and (iii) use of less volatile solvents such as ionic liquids (ILs) instead of volatile organic solvents. Over the last two decades ILs were proposed as serious contenders for CO2 capture. Since ILs can be tailor-made by combining cations and anions, task specific solvents can be produced.8 The most pronounced physical property of most ILs is their negligible vapor pressure. This would enable ILs to replace the conventional organic solvents. The drawbacks associated with ILs are their high price, complex purification resulting in large waste streams7 and since most of them are produced from fossil resources they are nonbiodegradable. Currently, we are exploring the potential of a new type of solvent, so-called low transition temperature mixtures (LTTMs), as sustainable substituents for the traditional CO2 absorbents. LTTMs are mixtures of solid compounds that form liquids upon mixing and have very low melting points, far below that of the individual compounds. In most cases, an LTTM exists of a mixture of a natural organic salt (acting as hydrogen bond acceptor, HBA) and a natural amide, organic acid or (poly)alcohol (acting as hydrogen bond donor, HBD), which can associate with each other via hydrogen bonding interactions. The main difference between LTTMs investigated in this work and deep eutectic solvents (DESs)9 is that the latter shows a first order phase transition (e.g., eutectic melting point), while the 4 ACS Paragon Plus Environment

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former shows a second order phase transition (e.g., glass transition point) upon cooling.10 While their physicochemical properties are similar to those of ILs, LTTMs are generally less toxic, biocompatible, much cheaper and easy to prepare, hence bypassing complex purification steps and waste disposal encountered with ILs. In this study lactic acid, a natural organic acid and a well-known preservative acting as HBD, is combined with tetramethylammonium chloride, tetraethylammonium chloride or tetrabutylammonium chloride, which are quaternary ammonium salts that have been investigated in numerous clinical and pharmacological studies (e.g., blocking transport of potassium ions11) acting as HBAs. The LTTMs are formed at molar ratio of HBD:HBA = 2:1. These LTTMs were chosen for carbon capture, because they have very low transition temperatures, allowing them to be used at industrial conditions without too high viscosities. The thermal stability and physical properties are experimentally determined. The phase behavior of CO2 with all LTTMs has been measured using a gravimetric technique, which is known to be a good method to accurately determine the absorption isotherms of gases in sorbents8,12. The Henry’s law constant and heat of absorption have been evaluated from the dependence of the solubility on pressure and temperature. To date, the number of studies involving LTTMs or DESs as alternative sorbents for CO2 capture is extremely limited.13,14,15 In these studies, the solubility behavior of CO2 in LTTMs and/or DESs has been determined, but despite the importance of the diffusion coefficients in the mass transfer of the CO2 absorption process, to the extent of our knowledge, no kinetic data of such a system has been published yet. Here, we study for the first time the diffusion coefficients of CO2 in LTTMs.



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2 2.1

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Experimental Preparation of the LTTMs

Tetramethylammonium chloride (TMA-Cl, ≥98%), tetraethylammonium chloride (TEACl, ≥98%) and tetrabutylammonium chloride (TBA-Cl, ≥98%) were purchased from Sigma-Aldrich. Ultra-pure crystalline L-lactic acid (LA) of pharmaceutical grade was kindly provided by PURAC Biochem B.V., Gorinchem, The Netherlands. The CO2 used for the measurements was supplied by Linde AG, The Netherlands, and had an ultra-high purity of 99.995%. In Figure 1 the molecular structures of the starting chemicals are shown.

Figure 1: Molecular structures of the constituents of the LTTMs.

Lactic acid was dried under vacuum in a desiccator. The mixtures were prepared by weight using an analytical balance (Sartorius Extended ED224S) with 0.1 mg readability. The HBD (lactic acid) and the HBA (quaternary ammonium salt) were added to a capped glass bottle and mixed thoroughly using a vortex mixer (VWR) to obtain a homogenous mixture before heating at 308 K overnight in the thermostatic oil bath with a temperature 6 ACS Paragon Plus Environment

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controller (IKA ETS-D5). The mixture is stirred during the heating using a magnetic stirrer. Subsequently, the formed LTTM is cooled to room temperature and stored in the vacuum desiccator until usage. Molecular weights of the HBD, the HBAs and the average molecular weights of the formed LTTMs are given in Table 1. The moisture (water) content in the prepared LTTMs was measured with the Karl Fischer titration method (795 KFT Titrino Metrohm Karl Fischer) and found to be less than 1 wt% in all cases. Table 1: Chemicals used in this study, including their source, purity and molecular weight

Component Lactic acid TMA-Cl TEA-Cl LA:TMA-Cl (2:1) LA:TEA-Cl (2:1) CO2

2.2

Source Purity Molecular weight (g.mol-1) Purac ≥99% 90.08 Sigma-Aldrich ≥98% 109.60 Sigma-Aldrich ≥98% 165.70 LTTM prepared by mixing ≥98% 96.59 LTTM prepared by mixing ≥98% 115.29 Linde AG ≥99.995% 44.01

Thermal operating window

The thermal stability of the samples is determined by using a thermogravimetric analyzer (TGA 4000, PerkinElmer). The LTTM sample was heated from room temperature to 650 K at a scanning rate of 5 K.min-1 in a ceramic crucible under a continuous nitrogen flow (30 mL.min-1) and a gas pressure of 0.2 MPa. In addition, the long term thermal stability of the LTTMs was investigated by running two isotherms at 323 and 333 K for 12 h. The weighing precision and sensitivity of the balance are ±0.01 % and 1 µg respectively. The temperature is accurately measured within 1 K. The glass transition temperature of the LTTMs was studied using differential scanning calorimetery (Pyris Diamond DSC, PerkinElmer). The operating temperature range of the DSC is 103.15 to 1003.15 K and the scanning rates range from 0.01 to 500 K.min-1. The measurements were carried out within a temperature range of 163 to 240 K at a heating 7 ACS Paragon Plus Environment


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rate of 5 K.min-1. The temperature measurement is accurate within ±0.1 K. The calorimeter precision and sensitivity are ±0.1% and 0.2 µW, respectively.

2.3

Physical properties

Measurements of density and viscosity were performed using an Anton Paar SVM 3000 Stabinger Viscometer, which determines both properties simultaneously. The viscometer has a dynamic viscosity range of 0.2 to 20 000 mPa·s and a density range of 0.65 to 3 g·cm-3. Viscosity and density were measured in the temperature range of 293.15 to 318.15 K at atmospheric pressure. The temperature uncertainty in the temperature range of the equipment (288.15 to 378.15 K) is ± 0.02 K and the relative uncertainty of the dynamic viscosity is ± 0.35 %, while the absolute uncertainty in the density is 0.0005 g·cm-3.

2.4

CO2 absorption capacity

The solubility of CO2 in all LTTMs was studied by determining the bubble-point curve using a magnetic suspension balance (MSB, Rubotherm GmbH), which is schematically shown in Figure 2. In the MSB isotherms are measured (i.e., Px-diagram, the pressure is increased stepwise and the CO2 loading is measured at constant temperature). A magnetic suspension coupling is used to transmit the force from the sample in the measuring cell to the microbalance. An electromagnet, which is attached to the bottom of the balance via a weighing hook, maintains the freely suspended state of the suspension magnet via an electronic control unit. The suspension magnet, which is used for transmitting the force, consists of a permanent magnet, a sensor core and a measuring load decoupling. Thus, with the MSB it is possible to weigh samples contactlessly in almost any environment with the microbalance located at atmospheric conditions outside the measuring cell. The MSB is a gravimetric apparatus that can operate in the pressure range from ultrahigh vacuum (UHV) to 15 MPa and in the temperature range from 273.15 up to 423.15 K. The 8 ACS Paragon Plus Environment

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maximum measuring load of the suspension balance is 25 g. The resolution of the mass reading is 0.01 mg. The temperature of the sample is regulated by an external circulating bath filled with silicone oil (Julabo FP50-ME). The temperature is measured using a platinum resistance probe (Pt-100). The Rubotherm analyzer provides accurate computer control and measurement of mass, pressure and temperature to correctly determine the gas absorption and desorption isotherms. The supplied control and data acquisition software MessPro regulates the magnetic suspension coupling and the peripherals (Julabo FP50-ME thermostat, Sartorius balance, Jumo imago 500 process control (Gas dosing system) and temperature control (hot chamber/tubes), Jumo Dicon 400 Q temperature indicator and Isco D260 syringe pump) within a pre-configured measurement program as well as in manual operation. Moreover, the MSB can operate in both static and dynamic modes. In the dynamic mode the gas dosing provides a continuous flow of gas passing the sample cell and the ventilation valve controls the set-point pressure. In the static mode, the gas is introduced to the measuring cell and both the admittance and the ventilation valve control the setpoint pressure. All the absorption measurements in this study were carried out in the static mode in order to minimize the aerodynamic drag forces created by the flowing gases. There are several distinct merits of the MSB worth to mention. First, the density of the gas surrounding the sample is measured with high accuracy. The gas density is needed to correct the buoyancy effect acting on the sample when the pressure deviates from vacuum in order to obtain reliable solubility and kinetic data. Second, the MSB can be tarred and calibrated during the measurements. Third, external conditions (e.g., ambient temperature, pressure and humidity) and changing conditions inside the measuring cell can cause zeropoint jumps and need to be corrected. This zero-point correction along with the calibration of the microbalance is possible and essential for measuring the gas solubilities and kinetics 9 ACS Paragon Plus Environment


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accurately, especially because it takes a long time before the vapor-liquid equilibrium is reached. Before the absorption measurements are initiated, the mass of the sample container in vacuum (msc) and the volume of the sample container (Vsc) have to be determined during a blank measurement. Here, an absorption isotherm is measured without sample using an inert gas (e.g., He). Starting from vacuum, the pressure is increased in small steps while the temperature is kept constant. The recorded data during the blank measurement are temperature, pressure and the balance reading (mbal). When the balance reading is plotted as function of the density of the reference gas (ρg_ref), the result is a straight line with negative slope. The negative slope is due to the buoyancy effect acting on the sample container. The linear relation reads:

mbal = msc − ρg _ ref ⋅Vsc

(1)

Thereafter, the LTTM was loaded into the sample container of the MSB and degassed by heating it at 308 K under vacuum. This is done for sufficient long time to reduce its content of water and volatiles. During this reactivation step, the mass of the loaded sample container in vacuum (msc+s) is measured. The mass of the degassed and de-watered sample (ms) can be calculated by: m s = m sc + s − m sc

(2)

During the absorption measurements the mass of the sample weighed must be corrected for the buoyancy effect acting on it by the measuring gas phase. This buoyancy effect is proportional to the product of the density of the gas surrounding the sample and the volume of the loaded sample container (Vsc+s·ρg). When the density of the sample is known (e.g., using the Anton Paar density meter), the volume of the sample can be calculated (Vs=ms/ρs). When the volume of the sample (Vs = Vsc+s - Vsc) (or density) is 10 ACS Paragon Plus Environment

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unknown, a buoyancy measurement must be carried out. The density of the sample can be calculated from its mass and volume (ρs=ms/Vs). The buoyancy measurement is analogous to the blank measurement. The main difference between the two is that in the latter the sample container is loaded with the sample. Furthermore, unlike the blank measurement, the buoyancy measurements must be performed at the temperature, at which the absorption isotherm is measured, to correct for the thermal expansion of the sample. The intercept of the linear relation between the balance reading (mbal) and the density of the inert gas (ρg_ref), is the total mass (msc+s) of the loaded sample container and its slope is the total volume (Vsc+s) of the loaded sample container:

mbal = msc+s − ρg _ ref ⋅Vsc+s

(3)

After the buoyancy measurement was performed, the sample and the MSB were evacuated before the isothermal absorption measurement was started. For solubility measurements of CO2 in all LTTMs, the gas valve was closed until the set temperature was established. Afterwards, gaseous or supercritical CO2 (100 mL.min-1) was introduced into the MSB and the pressure was increased stepwise until the set pressure was reached. The increase in the sample mass was recorded. Data treatment of the absorption measurements requires that the recorded balance readings (mbal) are corrected for the buoyancy effect:

mbal ,Corr = mbal + ρCO2 ⋅Vsc+s +CO2

(4)

where the buoyancy corrected mass (mbal,Corr) is the mass of the sample with the absorbed gas and the sample container, (Vsc+s+CO2) is the volume of the sample with the absorbed gas and the sample container and ρCO2 is the density of CO2 at the operating conditions. The mass of the absorbed CO2 (mCO2) is obtained by:



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(

)

mCO2 = mbal ( P, T ) − ( msc + s ) + Vsc + s + CO2 ⋅ ρCO2 ( P, T )

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(5)

Assuming that the volume of the absorbed gas is negligible, Eq. (5) reduces to:

mCO2 = mbal ( P, T ) − ( msc + s ) + (Vsc + s ) ⋅ ρCO2 ( P, T )

(6)

The CO2 mole fraction absorbed (xCO2) is determined from:

x CO2

 mCO2     M w,CO  2   =  mCO2   m s  +  M w,CO   M w, s   2 

(7)

   

where Mw,CO2 is the molecular weight of CO2 (44.01 g.mol-1) and Mw,s is the molecular weight of LTTM sample: 96.59 g.mol-1 for lactic acid + tetramethylammonium chloride (2:1) (LA:TMA-Cl (2:1)), 115.29 g.mol-1for lactic acid + tetraethylammonium chloride (2:1) (LA:TEA-Cl (2:1)) and 152.69 g.mol-1for lactic acid + tetrabutylammonium chloride (2:1) (LA:TBA-Cl (2:1)). After each absorption isotherm measurement, the LTTM was regenerated by heating at 308 K under vacuum until the mass was stabilized. Due to gas dissolution, the LTTM expands. Therefore, it is not correct to assume the volume of the sample (Vs) to be constant at different pressures. In order to make an appropriate buoyancy correction due to the LTTM sample volume change, mole fraction average for the molar volume may be used. This correction method is only applicable for low pressures ( 0.9999). The temperature-dependent binary interaction parameters are listed in Table 6. The binary interaction parameter kij was found to be slightly dependent on temperature, while lij was not temperature-dependent within the conditions applied. Therefore, the lij was kept constant and only the kij was varied with temperature. The negative values of the lij could be explained by the presence of repulsive forces within the liquid that leads to the expansion of the dissolved liquid phase. It is important to note that the use of binary interaction parameters means in general that the EoS is no longer fully predictive. However, it is inevitable to use binary interaction parameters to predict the phase behavior for mixtures consisting of molecules of very dissimilar sizes. Nevertheless, in order to use PR-EoS as a predictive model at various temperatures, the binary interaction parameters must be temperature-independent. For 35 ACS Paragon Plus Environment


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each of the studied systems (LTTM + CO2) the temperature-independent parameters are fixed and presented in Table 7. The modeling results are shown in Figure 16. It is evident from Figure 16 that fixing the values of the binary interaction parameters leads to larger deviations with the experimental data. Table 5: Molecular weight (Mw), critical properties (Vcm, Tcm and Pcm) and acentric factor (ω) of CO2 and the LTTMs investigated

LTTM LA:TMA-Cl (2:1) LA:TEA-Cl (2:1) LA:TBA-Cl (2:1) CO2

Mw/g.mol-1 96.59 115.29 152.69 44.01

Vcm/cm3.mol-1 285.91 352.87 475.05 94.07

Tcm/K 636.43 658.39 695.02 304.12

Pcm/MPa 4.02 3.30 2.47 7.374

ω 0.86 0.92 1.03 0.225

Table 6: Peng-Robinson modeling results including binary interaction parameter (temperature-dependent kij and fixed lij) and percent absolute average relative deviation (AARD%) from experimental VLE data

System

T/K

LA:TMA-Cl (2:1) + CO2

308.15 318.15

Number of data points 20 20

kij

lij

AARD%

0.203 0.210

-0.175 -0.175

2.62 2.08

LA:TEA-Cl (2:1) + CO2

308.15 318.15

20 20

0.198 0.210

-0.0112 -0.0112

2.01 2.37

LA:TBA-Cl (2:1) + CO2

308.15 318.15

14 14

0.167 0.179

-0.0544 -0.0544

1.74 1.20

Table 7: Peng-Robinson modeling results including temperature-independent binary interaction parameters (kij and lij) and percent absolute average relative deviation (AARD%) from experimental VLE data

Number of kij lij AARD%a data points 20 0.203 -0.175 2.62/7.27 LA:TMA-Cl (2:1) + CO2 20 0.198 -0.0112 2.01/8.09 LA:TEA-Cl (2:1) + CO2 14 0.167 -0.0544 1.74/6.47 LA:TBA-Cl (2:1) + CO2 a the presented deviations are for the two measured isotherms 308.15 and 318.15 K, respectively. The first value belongs to the 308.15 K isotherm and the second to the 318.15 K isotherm. System


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Figure 15: Experimentally determined isotherms for the system containing (▲) LA:TMA-Cl (2:1) + CO2, (●) LA:TEA-Cl (2:1) + CO2 and (■) LA:TBA-Cl (2:1). The closed symbols represent the experimental data at 308.15 K and the open symbols represent the data at 318.15 K. The solid lines represent the modeling results using PR-EoS with temperature-independent binary interaction parameters (values of kij and lij to be found in Table 6).



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Figure 16: Experimentally determined isotherms for the system containing (▲) LA:TMA-Cl (2:1) + CO2, (●) LA:TEA-Cl (2:1) + CO2 and (■) LA:TBA-Cl (2:1). The closed symbols represent the experimental data at 308.15 K and the open symbols represent the data at 318.15 K. The solid lines represent the modeling results using PR-EoS with temperature-independent binary interaction parameters (values of kij and lij to be found in Table 7).

4

Conclusions

In this study three new LTTMs have been prepared by mixing lactic acid (HBD) with tetramethylammonium chloride, tetraethylammonium chloride or tetrabutylammonium chloride (HBA) at a molar ratio of 2:1. The LTTMs formed stable liquids at room temperature. The mixtures were examined as potential solvents for CO2 absorption. A comprehensive characterization of the newly created solvents has been conducted. The thermal operating window has been determined, and physical properties have been reported. Furthermore, the solubility of CO2 and kinetics of CO2 absorption in the three LTTMs have been evaluated. The solubility of CO2 in the investigated LTTMs (up to 11.2 mol% at 308 K and 19.9 MPa) was found similar to that in other LTTMs studied so far, but lower than that in fluorinated ILs. Other important thermodynamic properties, including Henry’s law coefficient, the molar heat and entropy of absorption of CO2 in the new 38 ACS Paragon Plus Environment

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sorbents, have been determined. The diffusion coefficients of CO2 in the LTTMs were in the order of 10-11 to 10-10 m2.s-1, which are similar to values for CO2 diffusivity in ILs at the same conditions. Despite the low CO2 absorptive capacity, LTTMs show a tremendous tunability of the thermal-, physical- and chemical properties by changing the composition and the starting compounds. This gives the researchers additional degrees of freedom for further optimization of this class of solvents for carbon capture.



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Acknowledgements Financial support from the European Union FP-7-Energy project (IOLICAP) is gratefully acknowledged. We would like to thank PURAC Biochem B.V., Gorinchem, The Netherlands for providing lactic acid free of charge. We would also like to thank W. Weggemans for his help during the experiments.

Supplementary Information This section presents: (i) tables with numerical data on the density and viscosity of the LTTMs and their temperature dependence; (ii) the fitting parameters and root mean deviations for the fitting of these two properties; and (iii) experimentally determined solubilities and diffusivities of CO2 in the three LTTMs at two distinct temperatures and in the pressure range (0.1-2 MPa) This information is available free of charge via the Internet at http://pubs.acs.org


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Low Transition Temperature Mixtures as Innovative and Sustainable

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