In-line pH measurement solutions from Mettler Toledo offer an efficient and cost-effective method for the amine sweetening process commonly used for the removal of sour gases in petroleum refining and natural gas processing.
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), which must be reduced due to the corrosive and toxic properties of H2S, and the reduced caloric value caused by CO2. The two gases are usually removed by passing the sour gas through an amine solution in an absorber unit.
Sweet gas is extracted from the top of the absorber, and the amine solution collected at the bottom, which now rich in H2S and CO2, is passed to a regenerator unit where heat is used to strip the sour gases from the amine. ‘Lean’ amine is then fed back to the absorber. Given the cost of amine and energy required for generating heat, 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.
Typical methods employed for monitoring acid gas load in amine units include laboratory analysis of samples or in-line H2S monitoring. While lab analysis is time consuming and does not provide real-time data, in-line H2S analysers provide real-time data but are expensive to purchase, install and maintain, and do not measure CO2 content. A third alternative, in-line pH analysis is one that is not only cost effective, but also highly efficient.
Value of pH monitoring
The acid gas load of the amine solution can be easily measured by monitoring the pH upstream and downstream of the amine regenerator. 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 correlation between the pH value of the amine solution and acid gas loading must be determined. This can be done empirically by first measuring the actual acid gas content of the rich and lean solutions in the laboratory and then comparing the acid gas concentrations found in the laboratory with the process pH values and temperatures at the time of taking the samples.
Mettler Toledo’s M700 transmitters, featuring an internal flash memory card 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.
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 while a decrease in the pH level downstream of the regenerator will indicate that stripping is not sufficient or that amine in the solution is spent, requiring fresh amine to be added.
Mettler Toledo's InPro 4260i electrode is ideal for pH measurement 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 to minimise fouling of the sensor. Fitting a retractable housing, such as the InTrac 777e allows any maintenance work to be conducted without interruption to the process.
Mettler Toledo's Intelligent Sensor Management (ISM) technology 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, which can be effectively and cost-efficiently achieved through amine sweetening by employing an in-line pH measurement solution from Mettler Toledo.