4 Measuring Instruments
4.2 Viscosity Measurement
4.2.1 Rheometer
A rheometer from Anton Paar (Physica MCR 101) was used to perform viscosity measurements of all amine + H2O + CO2 mixtures. The dynamic viscosity was measured via a double-gap pressure cell XL measuring system that is recommended for the low viscous solutions since the probe provides a high surface area between the fluid and the probe [201]. The rheometer is built with an internal temperature controller that has a standard temperature uncertainty of 0.03 K. In order to measure viscosities at temperatures below 303.15 K, an external cooling system (Anton Paar Viscotherm VT2) has been employed with standard temperature uncertainty 0.02 K [150]. A sample of 7mL is fed into the gap between the fixed outer and inner wall of the rotating cylinder.
The rotating cylinder is rotated at fixed rpm in order to give a constant shear rate. The instrument measures the applied torque, shear stress and calculates the dynamic viscosity accordingly. A schematic of the double-gap pressure cell XL is given in Figure 4.2.
Figure 4.2: Schematic of double-gap geometry of the rheometer [202]
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Table 4.2 gives technical information about the double-gap pressure cell XL of the Physica MCR 101 rheometer.
The viscosity of liquid is determined according to the model as described below.
𝜏 = 1+𝛿2
(𝛿2𝑅32+𝑅22). 𝑇
4000𝜋𝐿 𝐶𝐿
(121)
𝛾̇ =𝜋𝑛
30.1+𝛿2
𝛿2−1 (122)
𝜔 = 2𝜋
60𝑛 (123)
Where 𝜏, 𝛾̇, 𝑇, 𝛿, 𝑅𝑖⁄𝑅𝑒, 𝜔 and 𝑛 are shear stress , shear rate, torque, radius ratio, internal/external cylinder radius, angular velocity and speed respectively.
Table 4.2: Technical information of the double-gap pressure cell XL.
Maximum Pressure 150 bar
Maximum Temperature 180 ℃
Concentricity of measuring systems ±0.01 mm Parallelism of measuring systems ±0.01 mm
It is important to select the proper shear rate in the rheometer to get an accurate viscosity of the amine solution. Figure 4.3 illustrates the relationship of viscosity vs shear rate that is provided by the manufacturer in which double-gap pressure cell XL denoted as DG35.12/XL/Pr (green box) is the valid measuring system for this work.
Figure 4.3: Viscosity and shear rate relation for different measuring methods [203]
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4.2.2 Shear Rate
The selection of shear rate was done by performing viscosity measurements at different shear rates of 200, 400, 600, 800, 1000 and 1200 s-1. The considered shear rates are within the range as shown in Figure 4.3. The viscosity was measured 10 times with 10 s time intervals at 303.15 K. The standard deviations of the measured viscosities at each shear stress were calculated. As given in Table 4.3, the standard deviation decreases with the increase of shear rate. A shear rate, which gives a low standard deviation, has to be selected for the viscosity measurements. At the CO2-laboratory in USN, 1000 s-1 of shear rate has been used for the viscosity measurement of amine + H2O and amine + H2O + CO2 mixtures and it has a low standard deviation compared to lower shear rates.
Accordingly, 1000 s-1 was selected to carry out viscosity measurements in this study.
Table 4.3: Variation of the standard deviation of viscosity measurements with shear rate.
Shear rate (s-1) Standard deviation (mPa⸳s)
30% MEA 40% MEA 50% MEA
200 0.0421 0.0345 0.0460
400 0.0243 0.0189 0.0235
600 0.0137 0.0088 0.0113
800 0.0061 0.0039 0.0059
1000 0.0029 0.0020 0.0059
1200 0.0010 0.0005 0.0055
4.2.3 Air check, Motor Adjustment and Calibration
The checking of the rheometer includes air check and viscosity measurement of a certified viscosity standard that is also known as standard oil. Air check and motor adjustment were performed prior to viscosity measurements to examine the quality of the motor adjustment and the conditions of the bearings. The residual friction in the bearing is measured during the motor adjustment. The measured values are saved and will be used in the viscosity measurements. Figure 4.4 shows the measured torque with deflection angle of air check before and after the motor adjustment.
Calibration of the rheometer is performed using a standard calibration fluid (S3S) provided by the Paragon Scientific Ltd. The standard oil is tested in accordance with ASTM D445. The uncertainties related with standard oil is given in Table 4.4 as provided by the supplier. The viscosity of standard oil was measured to identify the measurement error in the instrument and final viscosity measurements of all the amine solutions are corrected accordingly. Table 4.5 gives viscosities of the standard oil (S3S) as given by the supplier.
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Figure 4.4: First air check (green line) before the motor adjustment and second air check (blue line) after the motor adjustment.
Table 4.4: Uncertainties related with viscosity standard.
Viscosity range
Expanded Uncertainty Kinematic viscosity mm2s-1
(cSt)
Dynamic viscosity mPa⸳s (cP)
0.3 to 7.4 ± 0.07% ± 0.07%
7.4 to 10 ± 0.09% ± 0.09%
10 to 30 ± 0.12% ± 0.12%
30 to 72 ± 0.14% ± 0.14%
Table 4.5: Viscosities of the standard oil (S3S) as given by the supplier.
Temperature (○C) Viscosities of the standard oil / mPa⸳s
20.00 3.709
25.00 3.264
37.78 2.434
40.00 2.323
50.00 1.910
60.00 1.600
80.00 1.175
98.89 0.9161
100.00 0.9036
150.00 0.5339
4.2.4 Setting up Viscosity Experiments
The viscosity experiments in the rheometer were performed using the Rheoplus software (version 3.0x), which comes with the Physica MCR 101 rheometer. Figure 4.5 and 4.6 illustrate the details of the setup for viscosity measurements under constant
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The external cooling system was activated while the viscosity was measured as shown in a red circle in Figure 4.5. The temperature of the cooling system was set two degrees below the desired temperature as shown in the green circle. For the viscosity measurements at and above 303.15 K, only the existing temperature controller was employed as illustrated in Figure 4.6.
Figure 4.5: Setup for the viscosity measurements below 303.15 K.
Figure 4.6: Setup for the viscosity measurements above 303.15 K.
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4.2.5 Effect of Applied Pressure on Viscosity Measurements
Figure 4.7: Viscosity at 40% MEA 60% H2O mixtures at 303.15 K and P= 1 atm and P = 4 bar (N2 gas).
Figure 4.8: Viscosity at 27% AMP 3% MEA 70% H2O mixtures at 303.15 K and P= 1 atm and P= 4 bar (N2 gas).
For the viscosity measurements of CO2 loaded aqueous amine solutions, a pressure of 4 bar using N2 gas was applied to avoid possible CO2 and amine escape from the solution.
A test was performed to see the effect of applied pressure on the viscosity of the solution.
There, the viscosity of the sample was measured under atmospheric pressure and applied 4 bar pressure with N2 gas. The study shows the variation of viscosity due to the increase of pressure is less than 0.1% for the considered MEA + H2O and AMP + MEA + H2O mixtures. Figure 4.7 and 4.8 illustrate the effect of pressure on dynamic viscosity.
This indicates that in most cases the effect of pressure can be neglected.
0.003009
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