MODELING ADSORPTION IN SUPERCRITICAL FLUID · PDF fileMODELING ADSORPTION IN SUPERCRITICAL...

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MODELING ADSORPTION IN SUPERCRITICAL FLUID CHROMATOGRAPHY USING HIGH PRESSURE DROP COLUMNS Johannes Kern *, Monika Johannsen Institute of Bioprocess and Biosystems Engineering, Hamburg University of Technology, Denickestr. 15, 21071 Hamburg, [email protected], Fax: +49 40 42878 2909 An experimental approach using two columns is presented to measure pressure dependent adsorption equilibria in supercritical carbon dioxide and gain understanding of what is happening within high pressure drop chromatographic columns. This approach aims at creating an experimental method to check if pressure-dependent adsorption models can provide accurate interpolation if only data measured with a high pressure drop column are available. Adsorption equilibria of methanol on 2 µm silica particles at a temperature of 348.14 K and a pressure from 10.0 MPa to 13.4 MPa are presented and modeled using the Peng-Robinson equation of state (PR-EoS), the Real Adsorbed Solutions Theory (RAST) and a BET-type equation for pure methanol adsorption. All data can be described satisfactorily by the model using a common monolayer adsorption and BET α-term. The BET parameter , , which determines the locus of the asymptote is proposed to correlate with the solubility of methanol in supercritical carbon dioxide and is modeled using a two-parameter interpolation equation. INTRODUCTION One of the reasons why supercritical carbon dioxide is an attractive solvent for many processes is that its solvent properties can be widely varied by changing the pressure. This not only affects adsorption processes but also dynamic analytical methods that are used to measure adsorption equilibria, as the packed beds of adsorbent inevitably result in a pressure drop in the direction of flow [1]. When the adsorbent of interest is for example in the form of nanoparticles, very high pressure drops sometimes cannot be avoided. Therefore, thermodynamic models are needed to determine adsorption equilibria from experimental measurements even if the measured system contains pressure gradients that result in non- negligible density changes of the fluid. This work proposes an experimental method to acquire consistent adsorption data using a two column approach. MATERIALS AND METHODS Material MultoHigh U-Si unmodified silica columns, with a particle size of 2 µm, 125 Å pore size, were purchase from CS Chromatographie GmbH, Germany in the dimensions 250 x 4.6 mm and 50 x 4.5 mm. Methanol, 99.8 % purity UV/IR-grade, was purchased from Sigma-Aldrich, Germany. CO 2 , food grade, was purchased from YARA, Germany. REFPROP 9.1 fluid properties reference database was purchased from NIST, Boulder, USA.

Transcript of MODELING ADSORPTION IN SUPERCRITICAL FLUID · PDF fileMODELING ADSORPTION IN SUPERCRITICAL...

Page 1: MODELING ADSORPTION IN SUPERCRITICAL FLUID · PDF fileMODELING ADSORPTION IN SUPERCRITICAL FLUID CHROMATOGRAPHY USING HIGH PRESSURE DROP COLUMNS Johannes Kern *, Monika Johannsen Institute

MODELING ADSORPTION IN SUPERCRITICAL

FLUID CHROMATOGRAPHY USING HIGH

PRESSURE DROP COLUMNS

Johannes Kern*, Monika Johannsen

Institute of Bioprocess and Biosystems Engineering, Hamburg University of Technology, Denickestr. 15, 21071 Hamburg, [email protected], Fax: +49 40 42878 2909

An experimental approach using two columns is presented to measure pressure dependent adsorption equilibria in supercritical carbon dioxide and gain understanding of what is happening within high pressure drop chromatographic columns. This approach aims at creating an experimental method to check if pressure-dependent adsorption models can provide accurate interpolation if only data measured with a high pressure drop column are available. Adsorption equilibria of methanol on 2 µm silica particles at a temperature of 348.14 K and a pressure from 10.0 MPa to 13.4 MPa are presented and modeled using the Peng-Robinson equation of state (PR-EoS), the Real Adsorbed Solutions Theory (RAST) and a BET-type equation for pure methanol adsorption. All data can be described satisfactorily by the model using a common monolayer adsorption and BET α-term. The BET parameter ��,���� , which determines the locus of the asymptote is proposed to correlate with the solubility of methanol in supercritical carbon dioxide and is modeled using a two-parameter interpolation equation.

INTRODUCTION

One of the reasons why supercritical carbon dioxide is an attractive solvent for many processes is that its solvent properties can be widely varied by changing the pressure. This not only affects adsorption processes but also dynamic analytical methods that are used to measure adsorption equilibria, as the packed beds of adsorbent inevitably result in a pressure drop in the direction of flow [1]. When the adsorbent of interest is for example in the form of nanoparticles, very high pressure drops sometimes cannot be avoided. Therefore, thermodynamic models are needed to determine adsorption equilibria from experimental measurements even if the measured system contains pressure gradients that result in non-negligible density changes of the fluid. This work proposes an experimental method to acquire consistent adsorption data using a two column approach.

MATERIALS AND METHODS

Material

MultoHigh U-Si unmodified silica columns, with a particle size of 2 µm, 125 Å pore size, were purchase from CS Chromatographie GmbH, Germany in the dimensions 250 x 4.6 mm and 50 x 4.5 mm. Methanol, 99.8 % purity UV/IR-grade, was purchased from Sigma-Aldrich, Germany. CO2, food grade, was purchased from YARA, Germany. REFPROP 9.1 fluid properties reference database was purchased from NIST, Boulder, USA.

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Figure 1 Calculated pressure profile and experimental values. Data points correspond to measured column inlet pressures, line represents calculated pressure profile, z is the length coordinate from column inlet to outlet.

Frontal analysis

For the determination of adsorption isotherms, Frontal Analysis Chromatography was used: A concentration plateau of analyte is injected into the column and the time until inlet and outlet concentrations are the same is measured to calculate the adsorbed amount on the column. Adsorption isotherms of methanol on unmodified silica have been measured at 348.14 K and pressures from 10.0 MPa to 13.4 MPa. A novel experimental setup has been introduced. Two columns were used for the adsorption experiments with a bed length of 25 cm and 5 cm, respectively. The shorter column was used to represent five segments of the longer column, with segment 1 being the first segment at the column inlet. From preliminary experiments with the longer column, the pressure drop was determined with constant CO2 mass flow, temperature and column back pressure. These data were used to calculate inlet and outlet pressure pairs for five 5 cm segments inside the column, using Darcy's law and the REFPROP fluid reference database for CO2 viscosity and density. The pressure pairs in turn were used to set experimental conditions for the five measurements with the shorter column. This allows for a consistency check of the data, as the sum of the adsorbed amount in the segment experiments needs to be equal to the adsorbed amount in the longer column. Experiments were carried out at constant mass flow conditions of 1.3 g/min. For each isotherm, ten methanol concentrations ranging from 0.01 - 5.00 wt.-% have been injected into the column.

RESULTS

Calculation of pressure profiles

The pressure drop of the long column at 1.3 g/min CO2 flow, 10 MPa back pressure and 348.14 K was 3.4 MPa. To check the calculated pressure profile, the calculated pressures were compared to experimental ones from the shorter column at the same mass flow, temperature and fixed column inlet pressure. As can be seen in fig. 1, the experimental points agree with the calculated pressure profile within ±0.1 MPa accuracy. Adsorption experiments

The isotherms measured with the shorter column are depicted in fig. 2. Pressure dependent adsorption behaviour can be observed, which is more pronounced at higher methanol concentrations. For all pressures, adsorption increases with increasing methanol concentration without reaching a saturation adsorption, which suggests multilayer adsorption. Therefore,

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adsorption of pure methanol was described with a BET-type equation that was implemented into the model proposed by Wu et al. [2]:

��� ��,��

���

���,���� �� � 1���

���1 � ���� (1)

with the monolayer adsorption of component 2, ��,��, the maximum fugacity of pure component 2, ��,���

� , and the BET parameter, �. A more thorough explanation of the BET implementation can be found in [3]. As in the original model, pure CO2 adsorption was assumed to be of Langmuir type. Mixed adsorption was modeled according to RAST while bulk phase fugacities were calculated using the Peng-Robinson equation of state. All model parameters were fit to the whole dataset, with the exception of the BET parameter ��,���

� . This parameter determines the locus of the asymptote of the BET equation and is proposed to be correlated to the solubility of the adsorptive (in this case methanol) in the bulk phase. For supercritical carbon dioxide the solubility has been shown [4] to fulfill the relationship

��� ������� � (2)

With the solubility �, pure carbon dioxide density ���� and empirical parameters � and �. And under the assumption that � is proportional to ��,���

� it follows:

����,���� ������� �� (3)

Parameters � and b� were used as fitting parameters. This makes for 7 parameters that have to be fit to the experimental data, none of which are pressure-dependent. The values of these parameters can be seen in table 1. In fig. 3, the data of 2-propanol and methanol adsorption on C18 silica and plain silica, is shown, and the sum of the adsorbed masses on the 5 cm column experiments is compared with the adsorbed masses on the 25 cm column.

Figure 2 Adsorption isotherms of methanol on MultoHigh U-Si silica at 348.14 K and varying pressure. Symbols represent data points, lines represent PR-EoS/RAST/BET model with one set of parameters for all pressures.

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Table 1 Best fit parameters for the PR-EoS/RAST/BET model for methanol adsorption on MultoHigh U-Si silica at 348.14 K fit to all isotherms simultaneously.

� !"# ��

"# $%&# ��' � �� �

[mmol/mg] [mmol/mg] [-] [MPa] [-] [-] [-]

2.411 44.150 1.324 1.126e-8 1.365 -0.0173 0.0233

For 2-propanol, the values are in good agreement, which is an indicator for good consistency of the experimental adsorption data. For methanol, the sum of the 5 cm column experiments is systematically slightly lower than the data for the 25 cm column. However, in this system pressure dependency of adsorption is much more pronounced and at the, for supercritical fluid chromatography, relatively high temperature of 348.14 K the change in volumetric flow rate between the column segments is quite high. The volumetric flow rate changes from 3.5 ml/min in the first segment to 5.0 ml/min in the fifth. Nevertheless, the qualitative agreement of adsorption behaviour is very good.

CONCLUSION

It has been shown, that the two column approach presented here can be a valuable experimental technique for a consistency-check of frontal analysis adsorption data from high pressure drop column adsorption data. With the modified PR-EoS/RAST/BET model it is possible to model the adsorption data at different pressures with very high accuracy, especially considering the large change in pure CO2 density between highest and lowest pressure of about 40%. The proposed interpolation equation for the BET parameter ��,���

� works very well in the presented system in close proximity to the 2-phase region of the methanol-CO2 system.

Figure 3 left: adsorbed mass of 2-propanol on C18 silica at 35°C measured with a 25 cm column compared to the sum of adsorbed masses of experiments with a 5 cm column, right: adsorbed mass of methanol on plain silica at 348.14 K measured with a 25 cm column compared to the sum of adsorbed masses of experiments with a 5 cm column. � sum of 5 cm column experiments, � 25 cm column experiments, � BET model as visual aid

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REFERENCES [1] GRITTI, F., TARAFDER, A., GUIOCHON, G., Journal of Chromatography A, Vol. 1290, 2013, p. 73.

[2] WU, Y., WONG, D., TAN, C., Industrial & Engineering Chemical Research, Vol 30, 1991, p. 2492.

[3] KERN, J., JOHANNSEN, M., Journal of Supercritical Fluids, Vol 113, 2016, p. 72.

[4] KUMAR, S., JOHNSTON, K, Journal of Supercritical Fluids, Vol 1, 1988, p. 15.