Rapid continuous supercritical CO2 extraction and separation of

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Rapid continuous supercritical CO2 extraction and separation of organic compounds from liquid solutions Tatsuya Fujii, Yasuaki Matsuo, and Shin-ichiro Kawasaki Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00812 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Industrial & Engineering Chemistry Research

Rapid continuous supercritical CO2 extraction and separation of organic compounds from liquid solutions

Tatsuya Fujii*, Yasuaki Matsuo, Shin-ichiro Kawasaki* Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku Sendai, Miyagi 983-8551, Japan

*Corresponding author. Tel +81 22 237 8184 E-mail address: [email protected] (Tatsuya Fujii), [email protected] (Shin-ichiro Kawasaki) Postal address: 4-2-1 Nigatake, Sendai, Miyagi 983-8551, Japan

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Experimental apparatus was developed for rapid and continuous supercritical CO2 extraction and separation of hydrophobic organic compounds from liquid solution. The mixing of supercritical CO2 and liquid solutions in a micromixer enhances mass transfer, which enables rapid extraction of hydrophobic organic compounds. Near-equilibrium yields were obtained in about 10 s. The separation of supercritical CO2 from liquid was achieved using a newly developed separation system. In this separation system, the liquid level is controlled by the differential pressure between the head pressure and the supercritical CO2 phase. A control bulb at the outlet of the liquid line was regulated to maintain a constant differential pressure. The extraction of aqueous vanillin solution was conducted at 40 °C under pressures of 10, 15 and 20 MPa. Our results suggest that greater than 97% equilibrium was achieved in our system within the extraction time of 10 s. The extraction of a black liquor filtrate, that contains vanillin, its related compounds, and more than 5 wt% salts, was also conducted. More than 80% of the vanillin was successfully extracted to the CO2 phase.


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1. Introduction Batch-type processes have been widely used for the manufacturing of chemicals, especially fine chemicals. Recently, however, flow-chemistry has been gathering considerable attention, since the flow-process is considered more efficient and environmentally benign. For example, Tsubogo et al. reported the multistep, continuous-flow synthesis of rolipram, using heterogeneous catalysis [1], and Adamo et al. reported the on-demand, continuous-flow production of pharmaceuticals in a compact, reconfigurable system [2]. In both cases, however, the extraction and separation processes, needed more time and were batch-type in nature. Therefore, a rapid and continuous extraction/separation process is required. In bulk chemical processing, an environmentally benign and continuous extraction/separation process is also desired. For example, the black liquor filtrate produced as a byproduct in the paper manufacturing process contains natural vanillin. The extraction of vanillin is important because it is a well-known building block for chemical transformations [3]. Recently, reducing of the amount of organic solvents in the manufacturing of chemicals has been required. Supercritical CO2 has very similar physical properties (such as the dielectric constant, density, etc.) to that of hydrophobic organic solvents used. Therefore, supercritical CO2 has been considered a viable alternative. In this context, the extraction process using supercritical CO2 as the extracting solvent, i.e. supercritical fluid extraction (SFE), has gathered



much attention. Consequently, there have been various reports on the use of SFE and examples of the commercialization of SFE plants [4-6]. Investigations conducted using SFE mainly target solid materials, and batch or semi-batch (percolation) type processes. Recently, however, continuous SFE processes have also been studied. Domínguez et al. have reported continuous ethanol extraction from aqueous ethanol solution, using micro mixing devices [7]. They successfully

extracted equilibrated amounts of ethanol, in a few seconds, via enhanced mass transfer in a micro

mixing device. As suggested in the report, extraction using micro mixing device is promising.

However, their separation process is batch type, and so continuous flow out of the extract cannot be

conducted, which means that the process needs to be multi-tasked and will prove difficult to obtain a

high through-put. Ota et al. have reported on the continuous separation system of SFE. The system

uses two backpressure regulators, which function alternately by temporal difference [8,9]. However,

the flow rate is limited to low values and so it is difficult to realize high-throughput extractions,

which satisfy commercialized process.

In this study, we have developed a new separation system that can continuously flow out supercritical CO2 and liquid by controlling the liquid level. By combining this system with continuous extraction, using micro-mixing devices, we have succeeded in developing a continuous SFE/separation process. In this paper, we describe a continuous extraction and separation system. In addition, we show that this process is stable, and can be adapted to the


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SFE of vanillin from aqueous vanillin solution. Further, we have adapted this system for the extraction of vanillin from black liquor filtrate. The results are shown below.

Experimental The schematic diagram for the continuous SFE/separation process is shown in Fig. 1. A liquid solution and CO2 are separately pumped with high pressure pumps and mixed in a micro mixing device. The micro mixing device was a low-dead-volume tee (SWAGELOK Co. Ltd., i.d. 0.3 mm). At the tee, the two fluids interact and liquid-CO2 emulsion is formed. The emulsion has a large interface area and enhances mass transfer from liquid to supercritical CO2. The mixed fluid then enters a separation cell, where it separates into a CO2-rich phase (upper phase) and a liquid-rich phase (lower phase). The top of the cell and the bottom of the cell are connected separately to a differential pressure detector. The differential pressure observed is between the head pressure and the supercritical CO2 phase. Since the fluid level is directly proportional to the differential pressure, the fluid level can be controlled by the value of the differential pressure. The control bulb, in the liquid line, is regulated to maintain a constant value for the differential pressure. The separation cell is divided in two parts: separation occurs in the upper part and liquid level is controlled at the lower part. In general, the lower section is designed to be narrower in order to have increased sensitivity for the control of the liquid level. The system



pressure is controlled by a backpressure regulator in the CO2 line. Before depressurization, a solvent is introduced by a high-pressure pump to avoid precipitation by the depressurization of CO2. In this paper, water is used as the solvent and the extracts are obtained as aqueous solution. The extraction temperature was fixed at 40 °C and was controlled by a water bath. The extraction pressure is controlled by the backpressure regulator, as mentioned above, and kept at 10, 15, and 20 MPa. The flow rate of the liquid was fixed at 10 g/min, that of CO2 was 40 or 60 g/min, and that of water was 10 g/min. The Reynolds number for the liquid (simplified as water) and CO2 (simplified as pure CO2) in the mixer (i.d. 0.3 mm) was 2.8×104 and 4.3×103, respectively, at a pressure of 20 MPa and CO2 flow rate of 40 g/min. For simplicity, the Reynolds number was calculated separately. Those values suggest that the mixed fluids had turbulent-like flow. Effluents were sampled for 10 min at three times. As a model solution, 0.1 wt% of aqueous vanillin was used. In addition, black liquor filtrate was also investigated. The black liquor filtrate was obtained from a paper manufacturing company. The black liquor filtrate contained about 0.1 wt% of vanillin, its related compounds, and more than 5 wt% salts. The concentration of vanillin in the effluent was analyzed by a HPLC-UV-vis (SPD-20A, Shimadzu Corp.) equipped with an ODS column and a guard column (Shim-pack VP-ODS and Shim-pack GVP-ODS). The wavelength used for this analysis was 230 nm, and the column temperature was set to 30 °C. As eluents 0.1% aqueous formic acid


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solution and acetonitrile were used, and analysis was done in the gradient mode.

3. Results & Discussion First, the stability of continuous mixing and separation was tested at 40 °C and 20 MPa, using pure water. Fig. 2 shows the temporal variation of the system pressure and differential pressure. As shown in Fig. 2, the pressure and differential pressure were stable. The standard deviation of the differential pressure was 0.010 kPa. Here, the liquid level can be related to the differential pressure as: ∆P = ∆ρ ∙ h ∙ g (1) where ∆P [Pa=kg/m/s2], ∆ρ [kg/m3], h [m], and g [m/s2] are the differential pressure, density difference between liquid and CO2, liquid level and gravitational acceleration unit, respectively. Then, the resulting equation is, h=

∆P (2) ∆ρ ∙ g

so, when ∆P varies by 0.010 kPa, h varies by 6 mm, since ∆ρ = 160 kg/m3 at 40 °C and 20 MPa (the value of pure water [10] and pure CO2 [11] was used) and g = 9.8 m/s2. This suggests that the liquid level was controlled at a fixed level ± 6 mm in the experiment (1-sigma interval). Next, 0.1 wt% of aqueous vanillin solution was used as the model liquid. Experiments



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were conducted at pressures of 10, 15 and 20 MPa and at a temperature of 40 °C. The results are listed in Table 1. Standard deviations of each yield are also shown with values of yields. Those values are calculated from vanillin concentrations of three recovered solutions. Standard deviations of vanillin yields are less than 0.01, which indicates that the continuous SFE/separation system was stable. The degree of equilibrium is defined as the vanillin yield in CO2 divided by the equilibrated vanillin yield in CO2. This value is calculated based on the reported partition coefficients of vanillin in the CO2 phase [12].

Table 1 List for results of 0.1wt% of aqueous vanillin solution at 40 °C P [MPa]

vanillin yield

vanillin yield

in CO2 [-]

in liquid [-]

10

0.513±0.004

0.482±0.006

0.99

0.97

15

0.690±0.003

0.302±0.001

0.99

0.99

20

0.755±0.002

0.243±0.005

1.00

1.00

balance [-]

degree of equilibrium [-]

Table 1 indicates that over 99% of the mass balance is confirmed and the degree of equilibrium is over 97% under the defined conditions of the experiment. For these experiments, the residence time from the mixing point to the inlet of the separation cell is approximately 10 s. Therefore, near-equilibrium conditions were achieved within 10 s. The time required for equilibration is much shorter than that of a batch equilibrated system (time required is about 2 hours) [12]. We propose that this observation is probably because the Reynolds number, at the


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mixing point, is high enough to form a liquid-CO2 emulsion. The formation of an emulsion means that there is a large liquid-CO2 interface area, and thus a fast mass transfer of vanillin from liquid to CO2 was achieved. Extraction of the black liquor filtrate, produced from the paper manufacturing process, was examined. The flow rate of the black liquor filtrate and that of CO2 were set to be 10 and 60 g/min, respectively. Fig. 3 shows the temporal variation of the system pressure and differential pressure. As shown in Fig. 3, the system pressure and differential pressure were stable with the standard deviations with 0.079 MPa and 0.013 kPa, for the extraction pressure and differential pressure, respectively. From equation (2) the liquid level was considered controlled at a fixed value ±7 mm. Vanillin was successfully extracted and recovered as an aqueous solution; the yield in CO2 phase was 0.813. The results indicate that the newly developed, continuous SFE/separation apparatus, can be adapted to dilute model aqueous solutions without salts as well as to genuine industrial solutions that contain high salt concentrations and other organic compounds. The use of this apparatus to probe the effect of other solutions, their related salts, and additional compounds, on the extraction process are currently being investigated. The results will be reported and discussed in a future manuscript.

4. Conclusions



We have developed a process which enables fast and continuous extraction/separation of hydrophobic organic compounds in liquid, using supercritical CO2 as an extraction solvent. For vanillin extraction from an aqueous vanillin solution at 40 °C, 20 MPa, the extraction was equilibrated within about 10 s. Vanillin can be extracted in the CO2 phase, from a genuine industrial solution, with a yield of 81.3%. Here, a genuine industrial solution refers to the black liquor filtrate produced in the paper manufacturing process. This process is not restricted to only vanillin solutions and can be adapted for use with other aqueous liquids that contain hydrophobic compounds, examples of such solutions will be demonstrated in near future.

Acknowledgement This invited contribution is part of the I&EC Research special issue for the 2018 Class of Influential Researchers. This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Experiments and analysis were supported by Mr. Yosuke Nakagawa.

References [1] Tsubogo, T.; Oyamada, H.; Kobayashi, S. Multistep Continuous-flow synthesis of (R)- and


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(S)-rolipram using heterogeneous catalysis. Nature 2015, 520, 329. [2] Adamo, A.; Beingessner, R.L.; Behnam, M.; Chen, J.; Jamison, T.F.; Jensen, K.F.; Monbaliu, J-C.M.; Myerson, A.S.; Revalor, E.M.; Snead, D.R.; Stelzer, T.; Weeranoppanant, N.; Wong, S.Y.; Zhang, P. On-demand Continuous-flow Production of Pharmaceuticals in a Compact, Reconfigurable System. Science 2016, 352, 61. [3] Schmitt, D.; Beiser, N.; Regenbrecht, C.; Zirbes, M.; Waldvogel, S.R. Adsorption and Separation of Black Liquor-derived Phenol Derivatives Using Anion Exchange Resins. Separation Purification Tech. 2017, 181, 8. [4] Brunner, G. Supercritical fluids: Technology and Application to Food Processing. J. Food Eng. 2005, 67, 21. [5] Pereira, C.G.; Angela, M.; Meireles, A. Supercritical Fluid Extraction of Bioactive Compounds: Fundamentals, Applications and Economic Perspectives. Food Bioprocess Technol. 2010, 3, 340. [6] Chemat, F.; Rombaut, N.; Meullemiestre, A.; Turk, M.; Perino, S.; Fabiano-Tixier, A.-S.; Abert-Vian, M. Review of Green Food Processing Techniques. Preservation, Transformation, and Extraction. Innovative Food Sci. Emerg. Tech. 2017, 41, 357. [7] Domínguez, C.C.; Gamse, T. Process Intensification by the Use of Micro Devices for Liquid Fractionation with Supercritical Carbon Dioxide. Chem. Eng. Res. Des. 2016, 108, 139.



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[8] Maeta, Y.; Ota, M.; Sato, Y.; Smith Jr., R.L.; Inomata, H. Measurements of Vapor-Liquid Equilibrium

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Dioxide-Ethanol-Water Systems with a Newly Developed Flow-Type Apparatus. Fluid Phase Equilibria 2015, 405, 96. [9] Ota, M.; Sugahara, S., Sato, Y.; Y.; Smith Jr., R.L.; Inomata, H. Vapor-Liquid Distribution Coefficients of Hops Extract in High Pressure CO2 and Ethanol Mixtures and Data Correlation with Entropy-Based Solubility Parameters. Fluid Phase Equilibria 2017, 434, 44. [10] Wagner, W.; Pruss, A. The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use. J. Phys. Chem. Ref. Data 2002, 31, 387. [11] Span, R.; Wagner, W. A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures up to 800 MPa, J. Phys. Chem. Ref. Data 1996, 25, 1509. [12] Brudi, K.; Dahmen, N.; Schmieder, H. Partition Coefficients of Organic Substances in Two-Phase Mixtures of Water and Carbon Dioxide at Pressures of 8 to 30 MPa and Temperatures of 313 to 333 K. J. Supercrit. Fluids 1996, 9, 146.


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Fig. 1. Schematic diagram of the continuous SFE/separation developed in this study

Fig. 2, Temporal variation of the system pressure and differential pressure (40 °C, 20 MPa) for experiments using pure water and CO2



Fig. 3. Temporal variation of the system pressure and differential pressure (40 °C, 20 MPa) for experiments using black liquor filtrate and CO2


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Table of contents (TOC)/abstract graphics


Rapid continuous supercritical CO2 extraction and separation of

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