Measuring Amine Strength in Absorption and Regeneration Solutions Using Handheld Raman

Alkanolamines are polyfunctional molecules that include the properties of both alcohols and amines, and because of this they serve as key intermediates in the development of a wide range of products. These molecules play a major role in various industries, including fabric treatment, water treatment, gas treatment, pharmaceutical, and coatings. Monoethanolamine (MEA) and Diethnolamine (DEA) are utilized as a chemical intermediate in the manufacture of cosmetics, surface active agents, emulsifiers, and plasticizing agents.  Triethanolamine (TEA) is utilized as an intermediate in the manufacture of surface-active agents used in textile demulsifers, corrosion inhibitor, and as rubber chemical intermediate. Finally MethylDiethanolamine (MDEA) and Diglycolamine (DGA) are commonly utilized as a gas treating agent for absorption and removal of CO2, COS and H2S from hydrogen synthesis gas, refinery gas, and natural gas streams; selective solvent for recovery of aromatics from refinery streams.

Acid gases, such as H₂S and CO_2, are found in sour or raw gases. To generate pipeline quality natural gas, these naturally occurring gases have to be removed using a liquid absorption process. Contaminants including H₂S and CO₂ are often found in natural gas streams. When combined with water CO₂ produces carbonic acid, which is corrosive by nature. CO₂ also reduces the BTU value of gas, and in concentrations of more than 2% or 3 % the gas is unmarketable. H₂S is highly toxic and is corrosive to equipment. Such contaminants can be removed by amine sweetening processes, so that the gas can be easily transported and marketable. Amines are widely used by crude oil refiners and natural gas producers. This is to remove unnecessary compounds from natural gas and refinery streams to make the products safe and effective, for both domestic and industrial usage. Gas treating, also known as gas conditioning, utilizes amine solutions to absorb H₂S and CO₂.

Reversible chemical reactions take place between the acid gas and the amine solution. This results in a thermal generation of the "rich amine solution" to remove the H₂S and CO₂ gases. The regenerated "lean amine solution" can be reused for another cycle of acid gas absorption. During the removal of acid gas, thermal regeneration of the rich amine solution is the most energy intensive procedure. Figure 1 shows the amine gas treatment process. In most cases an excess amount of energy is used to produce the absorbing liquid, this is to ensure it meets the specifications set for pipeline gas.

Figure 1. Amine gas treatment process used in refinery and power plant.

It is important to track the quality of the amine solution, which helps to ensure the gas absorption process is optimized and working correctly. Continuous measurement of the absorbing liquid quality, particularly for the regeneration liquid, will be valuable in fossil power plants and in the natural gas process industry. Generally a lab titration is used to manually determine the amine solution assay used in the gas absorption process. The heat stable salts produced are determined by an ion chromatography method, which can be time-consuming. In contrast, spectroscopic techniques are direct and provide results in real-time, therefore they can be used with a minimal time lag. Although, the aliphatic amines are not UV active and standard spectroscopic absorption methods, such as IR, UV, or NIR, are not of much use.

Raman spectroscopy provides instant response, which helps in predicting the composition of amine solutions due to the weak Raman scattering characteristic for water. Raman measurements take less than a second, and provide an alternative for a rapid quality control check of amines used in the gas absorption process. Creating a library with varied concentrations of amines, Raman spectroscopy can be used as a rapid screening tool to measure the assay of certain amines quickly and easily. The analytical capability, combined with the speed of analysis, is unmatched when compared to chromatography or wet chemistry technique.

Raman Instrumentation

Raman, FT-IR and other lab-based spectroscopy methods demand the same workflow, of sampling, labeling and transferring the sample to the analytical laboratory. Traditional lab-based Raman instruments can be costly and are mainly used by spectroscopists for reasearch and development purposes. Portable, Raman system provides a new analytical technique for main stream applications. However, many handheld systems use closely related sampling designs, which make it difficult to detect and qualify samples in field applications. This issue can be resolved by using the unique Orbital Raster Scan (ORS) sampling with its handheld systems. Other Raman instruments use a focused beam to create high spectral resolution, but they tend to overlook the analyte completely (A in Figure 2). Although a large beam in a dispersive system may help in collecting more scattered light, it would lead to a loss of spectral resolution. The Metrohm Instant Raman Mira’s ORS sampling technology extends the measuring area (C in Figure 2), as the laser beam travels over an expanded region of the sample surface, while making continuous measurements at various points and averaging them. This improves the precision, reproducibility, and the reliability of the measurement. ORS keeps the tightly focused beam, and boosts the interrogation area with a raster system. The large interrogation area enhances the signal strength, and the tightly focused beam preserves the high resolution. The combination of these two benefits produce a confident identification from a completely handheld system.

Figure 2. Orbital Raster Scan

The Mira analyzer offers the most adaptable sampling options, three quick change inserts provide rapid exchanges within the base system. These options include an integrated vial holder, where samples kept in vials are placed into the Mira for measurement. Vial measurements are appropriate for solid and liquid samples. When the cover is closed, laser radiation is prevented from emerging, this covering includes a safety feature that halts the measurement and turns off the laser in case it is opened. Mira, with the built-in vial holder, is the only portable Raman instrument that eliminates the need for laser safety goggles. Two point-and-shoot adapters allow the measurement of liquids, powders, and granulates in glass containers or polybags. They sustain the accurate optical alignment required to guarantee high-quality data, and can also accommodate several types of samples for material identification and verification purposes. The long focal length point-and-shoot adapter can be used to make measurements in colored glass bottles.

Mira can simplify quality control measurements as it brings the measurement to the sample directly, preventing the necessity for transferring samples to an analytical lab. It also eliminates the need for sample pre-treatment or any direct contact with the sample. It can test a sample in a fraction of seconds via a transparent packing material such as plastic bags or glass, and this significantly reduces/prevents the errors caused by sampling and sample preparation. Operators can easily interpret a Raman spectrum using the on-board automatic matching software. Mira comes with built-in system intelligence and search techniques which, enables the user to focus on results, and uses the spectral correlation vales. Simple and easy instrument operation steps eliminate the need for formal chemistry education in order to use the system on a regular basis.

Mira spectrometers are capable of capturing spectra of customers’ samples and using them to produce a comprehensive library. Thanks to the open library structure, users have the maximum flexibility to develop and use their own libraries. More than 400,000 samples can be stored in the library. With Mira’s fast scan ability, users can quickly develop their own library without any additional software. As soon as the instrument has been loaded with a library of the materials to identify, MIRA quickly does just that. This makes the Mira handheld Raman analyzer suitable for verifying incoming raw materials, or for qualifying the final products.

Experiment

Samples such as Monoethanolamine (MEA), Diethanolamine (DEA), Triethanolamine (TEA), and MethylDiethanolamine (MDEA) were used in the experiment. The instruments used were the Mira M-1 Advanced Package (now superseded by the Mira DS) and Mira Cal software. First, samples were transferred to the glass vial and then placed into the vial holder, the default measurement time was 0.5 seconds. A set of aqueous amine solution was prepared using water and MDEA. Bicarbonate solution was then used to acidify a diluted MDEA amine solution, the resulting acidified amine solution is measured to study the change in spectral patterns due to carbonate absorption.

Results and Discussion

Amine Selectivity: Identification

Raman spectroscopy allows the highest selectivity for primary, secondary, tertiary, aromatic, and aliphatic amines. Gas chromatography is often used to determine the purity of aromatic amines. Since aliphatic amines are highly polar and water-soluble compounds, their purity is often measured through titration or through gas chromatography following derivatization. However, these methods are tedious and lengthy. In contrast, Raman spectroscopy can be used to identify these amines quickly and easily. Figure 3 shows the overlay of the amines, MEA, TEA, DEA, and MDEA, and Table 1 lists the spectral correlation values.

Figure 3. Spectral overlay of common Ethanolamine used in Refinery and Power plant

Table 1. Specificity of amines – Spectral correlation values

With the library spanning the predicted types of amines, the Mira analyzer can be installed on the receiving area or production area, or it can be utilized as a powerful identification tool to track the quality of incoming amines.

Amine strength measurement: Confirmation

Discovering the amine solution concentration or amine strength is an important factor for plant processing. A weak amine solution will not effectively remove the acid component from the sour gas, so knowing the strength of the pure amine before it enters the absorber is vital. The amine strength can be measured with the Raman spectra signal. A set of aqueous amine solution was made with MDEA and water. The weight % of MDEA in the solution is 0.03 % to 99.5 %. Figure 4 shows the overlay of spectra with the varied MDEA concentration. When the amine strength increases, the intensity of the amine peak (1470^-1) also increases considerably.

Figure 4. Spectral overlay of varying concentration of MDEA.

For specific amine spanning the predicted concentration range a library can be built. Amine solutions entering the absorber can be tracked for the amine strength against their library spectra. The correlation value acquired for the amine solution against the standard solution in the library will provide a direct correlation to the purity of the amine solution. If the newly prepared/received amine solutions have reduction or contamination in concentration it will cause a decrease in the correlation value, which is proportional to the concentration or contamination level. The spectral correlation values for different concentration of the amine solutions are summarized in Table 2.

Table 2. Amine strengths – Spectral correlation values

Adding more samples that cover the predicted the concentration range to the library, will lead to improve prediction accuracy. Samples can be quickly studied for the purity, since the library spans the predicted concentration range from low to high.

Process Monitoring: Screening

Different types of amines have their own characteristic Raman spectrum. The Raman spectrum changes as these amines react with H₂S and CO₂. This change is due to a newly formed species or a change of the amine. The free amine then reacts with CO2 and H2S to form an ionized amine (RNH₃₊), Carbamate (RNHCO_2-), hydrogen carbonate, carbonate and HS-. These species have their own characteristic Raman spectrum. It is possible to monitor the total purity of the amine via the spectral correlation values, and the can be related to the amine purity. The overlay of the pure amine spectra with their acidified amine counterparts is shown in Figure 5.

Figure 5. Spectral overlay of pure amine and amine with acidic bicarbonate.

The spectral correlation values are also compared in Table 2. This correlation technique will track the strength of the "rich" amine solution to measure the time taken to replenish the amine solution. It is a fast yet simple method and can be used to determine the right time to stop the energy intensive "lean" amine solution regeneration process.

Conclusion

Handheld Raman method is suitable for estimating the strength of MDEA in an aqueous amine solution. Mira can be reliably used for testing aqueous alkanolamines for gas stripping process applications. This is because Mira measurements are fast and consistent when compared to the standard wet chemical or chromatography methods. Therefore, the Mira analyzer can be utilized as a quality assurance tool for checking the amine strength at the start of the absorption process as well as for tracking the rich amine process. It can even be employed as a confirmation tool to check the energy intensive lean amine solution regeneration process.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.