Amine sweetening is a common unit process for the removal of sour gases in petroleum refining and natural gas processing. However, it can be a costly procedure that is often inefficient. Real-time, in-line pH analysis ensures sweetening is effective and money is not being wasted.

Fuel gases and off-gases in refinery operations as well as all natural gas contain a certain amount of acidic gases in the form of hydrogen sulphide (H2S) and carbon dioxide (CO2). Due to the corrosive and toxic properties of H2S, and the reduced caloric value caused by CO2, the levels of these gasses must be curtailed if the products are to be used or processed further. The removal of H2S and CO2 is usually achieved by passing the sour gas through an amine solution in an absorber unit. "Sweet" gas is extracted from the top of the absorber, and amine solution collected at the bottom and now rich in H2S and CO2, is passed to a regenerator unit where heat is used to strip the sour gasses from the amine. "Lean" amine is then fed back to the absorber. The cost of amine and energy for generating heat can be substantial; therefore, refinery operators are keen to find a measurement control solution that ensures their amine sweetening process is effective as well as cost-efficient.

Controlling amine sweetening
The removal of H2S and CO2 from sour gases by amine sweetening relies on maintaining the quality of the amine solution fed to the absorber and proper adjustment of heat at the amine regenerator. Insufficient heat results in incomplete amine regeneration, while excessive heat represents a costly waste of energy. Feeding the absorber with an amine solution that is not sufficiently lean means gas absorbance will not be optimal, and adding fresh amine when it is not required is simply throwing money away.

Typical methods employed for monitoring acid gas load in amine units are laboratory analysis of samples or in-line H2S monitoring. Both techniques are not without their problems. Lab analysis is time consuming and does not provide real-time data. The amine sweetening process is a continuous operation and the acid gas load of the feed stock can fluctuate dramatically. Therefore, basing process control on grab sample analysis can be ineffective. Although in-line H2S analyzers do provide real-time data, they are expensive to purchase, install and maintain, and do not measure CO2 content. But there is a third alternative: one that is not only cost effective, but also highly efficient - in-line pH analysis.

Value of pH monitoring
Simply by monitoring the pH upstream and downstream of the amine regenerator, the acid gas load of the amine solution can be easily measured. When the acid gas load is known, the heat at the regenerator can be adjusted to the point where it efficiently removes the acid gases from the amine solution. Degeneration of the amine solution is also simply detected, so fresh make-up amine can be added in order to ensure sufficient absorption.

Before making adjustments to the process, the exact relationship between the pH value of the amine solution and the acid gas loading must be ascertained. The easiest way of doing this is empirically. First, the actual acid gas content of the rich and lean solutions needs to be measured in the laboratory by analyzing grab samples. Then, by comparing the acid gas concentrations found in the laboratory with the process pH values and temperatures at the time of taking the samples, the correlation between pH and acid gas loading can be determined.

The use of a METTLER TOLEDO M700 transmitter can be of great help here. With its internal flash memory card, the M700 can log both pH and temperature values for an extended period, making it easy to look up the process values at the time the samples were taken. Only one M700 is required for both the upstream and downstream pH electrodes.

Now, based on the upstream real-time pH level, adjustments at the regenerator can be made to ensure the optimum amount of heat is continually being used. A decrease in the pH level downstream of the regenerator indicates that stripping is not sufficient or that amine in the solution is spent and that fresh amine must be added.

Rugged electrodes
For the pH measurement itself, METTLER TOLEDO's InPro 4260i electrode is ideal as it is designed for extended life in harsh chemical environments. The InPro 4260i benefits from a solid electrolyte and an open junction instead of a diaphragm so that fouling of the sensor is minimized. Fitting a retractable housing, such as the InTrac 777e, allows any maintenance work to be conducted without interruption to the process.

Additional benefits come from the inclusion of METTLER TOLEDO's Intelligent Sensor Management (ISM) technology. ISM functionality includes Plug and Measure for fast, error-free start-up, and on-board diagnostics that predict when maintenance will be required.

Removal of H2S and CO2 from refinery and natural gases is a vital process in sour gas management. Achieving this through amine sweetening is effective and cost-efficient, providing that the amine solution used is sufficiently lean and that excessive use of heat at the regenerator is avoided. An in-line pH measurement solution from METTLER TOLEDO will not only verify that the amine sweetening process is doing its job, it is one of the simplest means of analyzing acid gas load, and ensuring that energy at the regenerator is not being wasted or that fresh amine is being misapplied.

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About Mettler Toledo:
METTLER TOLEDO specializes in the area of precision instruments for professional use. METTLER TOLEDO laboratory instruments are used in research, scientific, drug discovery, and quality control labs, amongst many others in the pharmaceutical, chemical, food and cosmetics industries. METTLER TOLEDO comprehensive industrial solutions cover the various steps in a host of manufacturing processes at many of the same customers that laboratory serves. Solutions range from receiving raw materials through various manufacturing processes, in-line process control and end-of-line packaging control, to logistics and shipping. Increasingly, these solutions are fully integrated into the customer's IT environment, helping automate their workflows. In food retail, our offering for fresh food management ranges from receiving and pre-packaging, to in-store solutions for self-service departments, deli counters, and checkout terminals.

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