Why is it difficult to achieve online monitoring + automatic control of injection with three injections?
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What are the practical difficulties? I hope those who have experience in actual production can offer some guidance
Let’s discuss this: when adjusting the dosage of neutralizing agents (amines or ammonia) in various refineries, is it based on online pH monitoring or offline monitoring? And how is the amount to be adjusted determined?
For injections, a metering pump is generally used to manually adjust the flow rate.
Neutralizing agents are generally ammonia or sodium hydroxide; of course, amines can also be used, though it is somewhat wasteful. I think it’s completely possible to achieve closed-loop control; pH meters are already very mature, and those that are maintained regularly still provide accurate readings. Amines are used as corrosion inhibitors; to achieve closed-loop control, a corrosion probe is required. I don’t think closed-loop control is very necessary, as it also carries risks. The amount of corrosion inhibitor used should be slightly excessive – for example, if the iron ion concentration drops below 3, 5 PPP of the inhibitor is sufficient, but in practice 10–15 PPM should be used.
Indeed, it is difficult to achieve automatic control of corrosion inhibitors, as the fluctuations detected by current probes are quite large and hard to manage. Fe ions are monitored offline, only once a week; as a result, it’s tough to adjust the amount of corrosion inhibitor to be injected based on these measurements. pH control seems feasible, but actual products always have various issues. Why is closed-loop control considered unnecessary? What major problems could arise from fully automated systems?
The purpose of closed-loop control is edge control, to achieve the lowest amount of corrosion inhibitor while ensuring no corrosion occurs. As a result, this leads to a reduction in control margin and an increase in risk. To achieve closed-loop control while reducing risks, it is necessary to use proper measuring instruments. The additional cost associated with using an instrument with a precision of 5 PPM must be compared against the cost of the instrument itself; it isn’t necessarily a cost-saving measure.
It seems that as long as Fe ions cannot be monitored in real time, it is difficult to determine the corrosion rate solely by using probes. I’m thinking about whether it would be possible to make the injection of ammonia and water automatic – this would certainly be more precise than manual control. The metering pumps could also be set to be operated manually, thereby allowing a combination of online and offline control. I wonder if it’s possible to achieve greater precision in water injection; perhaps raising the temperature of the water to 80-90 degrees could help slow down corrosion and keep the areas affected by dew point corrosion within a certain range of the pipeline
Ammonia injection must be subject to closed-loop control in the plant; too much injection leads to emulsification, while too little causes corrosion. The amount to be poured in is just right. There’s no need to raise the water temperature; corrosion inhibitors are meant to form a protective film, and the water temperature is already below the dew point. I’m really not sure about this question; let me think about it.