**3. An experiment on developing a full calibration model using Raman spectroscopy**

The scope in this section is to present the methodology on how a process analytical instrument is implemented to replace traditional chemical analysis. The example described in this section is specific to CO<sup>2</sup> absorption process by amines, but a similar approach can be applied for any chemical analysis in laboratory or process plant. Analysis of CO<sup>2</sup> capture solvents is necessary almost in every R&D tasks and CO<sup>2</sup> process plants to optimize the absorption and desorption process. Some examples are investigating the effects of different types of solvents, blends, catalyzers, process parameters and equipment configuration on CO<sup>2</sup> absorption and desorption capacity.

In this example, it is shown how a Raman spectroscopy-based full spectrum calibration is performed using a laboratory experiment for four types of amines which are reacted with CO2 . Sample types are described in **Table 1**. Two primary and two tertiary amines were used for preparing four regression models. The primary amines are 2-aminoethanol (MEA) and 3-amino-1-propanol (3-AP). Tertiary amines have a different chemical mechanism than primary and secondary amines when absorbing CO2 and they can reach CO2 loading capacity up to 1 mol CO2 /mol amine. 3-dimethylamino-1-propanol (3DMA1P) and methyl diethanolamine (MDEA) are the two tertiary amines used in this experiment.

### **3.1. Sample preparation**

are the main expectation from a PAT tool. The method of model development is described

procedure should follow the guidelines specified in *Theory of Sampling (ToS)* [16] and *Design* 

Element 4: Data processing and chemometrics are a subcategory of model development stage. When it comes to the implementation stage of a process analyzer to the plant, the researcher has experience from laboratory experiments or batchwise experiments on what kind of data treatment is needed. Usually raw data coming out from a process analyzer such as a spectrometer contain noise. The information in the data is hidden within this noise. Preprocessing makes data easier to read, understand and interpret. There are also some instances where univariate methods are implemented and/or data processing is not essential. After preprocessing, the data can be used as input to a chemometric model such as a principle component analysis (PCA) model or partial least square regression (PLSR) model. Different chemometric tools have different advantages. For example, PCA can be used to identify trends of variation in the data with respect to time or process conditions, and PLSR can be used to predict process features based on indirect measurements. PLSR is widely used in process applications for quantitative purposes. One such example can be found in [17] where it shows the use of

ous amine. Another example of PLSR model development and ion speciation is shown in [18]

Element 5: Data acquisition and instrument control of a process analyzer is an essential part to integrate analyzer measurements with an automative control system. In the case of univariate analysis, implementation of automation control system is easy. However, when it comes to multivariate methods, communication between the process analytical instrument and the control system is challenging. There is a gap between these two types. The reasons are that the analyzer has different file formats and sometimes the control system has been implemented in a different file format in a different flat form. For example, in the analyzer, the data may be saved in csv file format and the control system implemented in MATLAB/LabVIEW interface needs data only from a wavelength range and needs data in txt or mat format as inputs.

The scope in this section is to present the methodology on how a process analytical instrument is implemented to replace traditional chemical analysis. The example described in this section

process. Some examples are investigating the effects of different types of solvents, blends, catalyz-

absorption process by amines, but a similar approach can be applied for any

process plants to optimize the absorption and desorption

**3. An experiment on developing a full calibration model using** 

chemical analysis in laboratory or process plant. Analysis of CO<sup>2</sup>

ers, process parameters and equipment configuration on CO<sup>2</sup>

capture by ammonia process. A detailed description for PLSR theory, calibration and

capture process. The calibration and validation

capture laboratory rig operated by aque-

capture solvents is necessary

absorption and desorption capacity.

under Section 3 for an application in a CO<sup>2</sup>

192 Carbon Dioxide Chemistry, Capture and Oil Recovery

PLSR to quantify all chemical ions present in a CO<sup>2</sup>

validation is described in the literature [19, 20].

*of Experiment (DoE)*.

for CO<sup>2</sup>

**Raman spectroscopy**

almost in every R&D tasks and CO<sup>2</sup>

is specific to CO<sup>2</sup>

For each model, a calibration and validation set was prepared to facilitate *test set validation* [21]. Number of samples in each calibration and validation set is given in **Table 1**. All the samples were prepared using analytical grade chemicals and Milli-Qwater (18.2 MΩ cm). Aqueous solutions and water were degassed using a rotavapor. First, two amine stock solutions having 30 w/w% (weight per total weight of solution) concentration were prepared and stirred in closed containers for 30 min using mechanical stirrers. Stirring helps to mix the two phases of water and solvent to get a homogeneous solution. CO<sup>2</sup> was bubbled into one stock solution until the whole amine sample was *maximum CO<sup>2</sup> loaded*. The meaning of *maximum CO2 loaded* is that the solution is filled with equilibrium solubility of CO<sup>2</sup> at a given temperature and pressure. The time required to fully load the amine solution was calculated based on the data of volume of 30 w/w% amine sample, CO<sup>2</sup> bubbling flow rate, room temperature, pressure and equilibrium solubility. The CO<sup>2</sup> loaded amine solution was then stirred for 30 min in a closed vessel and left for 24 h at room temperature. The aim was to facilitate CO2 gas dispersion homogeneously throughout the solution; to accelerate the reaction between gas and solvent and to reach equilibrated state. One gram from each stock solution was transferred to a beaker to titrate with 1 M hydrochloric acid (HCl) to determine amine


**Table 1.** Description of samples in the calibration and validation set.

concentration. By mixing different ratios of CO<sup>2</sup> loaded amine solution with the other amine stock solution (*CO2 unloaded*), a series of 38–42 different CO<sup>2</sup> loaded samples were prepared in 10 mL glass reactors. After the solution in each glass reactor reached equilibrium, a titration method (refer Section 3.2) was carried out to measure its true CO<sup>2</sup> concentration in units of moles CO2 per mole solvent.
