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How a phosphate plant uses ultrasound metering to improve safety and production

Feb. 23, 2016
The use of ultrasound metering helped minimize acid leaks and achieve real time production control.

When you oversee a manufacturing process that involves a corrosive fluid such as sulfuric acid, the biggest danger is always leaks. Such was the case at our Mosaic fertilizer plant in Riverview, Florida.

Mosaic is the world's leading producer concentrated phosphate, a key ingredient in most fertilizers. We operate in every phase of phosphate development from the mining of the rock phosphate to the actual production of the phosphate fertilizer. And it’s in that production process that
sulfuric acid leaks are always a risk.

Phosphate production

To create phosphate suitable for fertilizer, first the phosphate rock is crushed, mixed with water and then combined with sulfuric acid in a reactor to produce phosphoric acid. Finally, the phosphoric acid is reacted with ammonia to produce ammonium phosphate fertilizer. Sufuric acid is produced on site from molten sulfur and water.

It is in the production of the sulfuric acid that the greatest risks of leaks occur. With the phosphate/sulfuric acid process it is critical that both the flow of the acid and its concentration be carefully monitored. The problem was that the meters we were using were intrusive and made contact with the acid flow.

Monitoring flow

We were using temperature compensated conductivity meters, also called titrimeters, to measure the concentration of the sulfuric acid. They provide accurate information when they’re working properly, but due to the fact that they are in direct contact with hot sulfuric acid, they are prone to failure. Even more than instrument failures, acid leaks are the biggest concern due to the safety and environmental impact they have. The conductivity meters were on a slip-stream of acid from the main recirculation flow and had been prone to frequent failures. And, since measuring acid concentration is critical to production, line failures would requires us to shut down to repair the leak. Needless to say, we were definitely looking for a way to either eliminate or reduce how often we have to send flow through these smaller lines.

Non-intrusive ultrasound

About two and a half years ago, I mentioned my problem to our account executive for Solares Controls, a manufacturer’s representative and distributor of measurement, fluid transfer, and process flow control equipment.

They had been supplying Mosaic with equipment for years. They recommended ultrasonic meters. We had already installed several ultrasonic flow meters from FLEXIM on our water and gypsum lines to measure strictly flow and we were pleased with the accuracy. They told me the same manufacturer also made meters that measured concentration and temperature as well as flow. Because ultrasonic meters are non-intrusive, I was immediately interested.

How ultrasonic works

The technique most ultrasonic flow meters use is called transit-time difference. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, kind of like a swimmer swimming against the current. The signal moves slower against the flow than with it.

When taking a measurement, the meter sends ultrasonic pulses through the medium, one in the flow direction and one against it. The transducers alternate as emitters and receivers. The transit time of the signal going with the flow is shorter than the one going against. The meter measures transit-time difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe.

Ultrasonic meter and concentration measurement

For concentration measurement, the FLEXIM PIOX® S uses the data gained from the ultrasound transmission technology to perform concentration analysis during process control and to calculate the (concentration compensated) mass flow rate. It is also useful for supervision of crystallization or phase separation processes for proper product identification and to determine density, yield rate, Brix or other customer specific measurement values.

Safety and better accuracy

The main reason we were looking at ultrasonic meters was safety. If we could eliminate the side streaming and intrusive metering, we could eliminate a lot of risk for our employees. As far as measurement is concerned, I’m more interested in acid concentration than flow.

Installation and test

I decided to take the cautious route and install one meter and monitor it for accuracy for a year. At the end of the year, I was satisfied with the way it tracked and saw that I might be able to eliminate most of our leak problems by getting concentration readings on the main acid lines. I ordered a second meter for a second line. After that worked out well, I decided to put them on all of our IPA lines. We now have 20 ultrasound meters working well and sending data back in real time.

Improved safety, accuracy, and production

I’m pleased at how non-intrusive metering makes job easier and the plant more efficient by reducing down time. Before we couldn’t repair acid leaks on the run because we couldn’t run without a concentration meter. Now, even if we only use the ultrasound as a backup to measure concentration, then we can make the repairs on the run and not have to shut down.

The future -- better production control

So far, the facility now has 20 FLEXIM PIOX meters installed, many as backups to the titrimeters, but a few as primaries. As they continue to prove their value, I plan to phase them in as primaries and eliminate the need for intrusive metering.

Accurate measurement of product production tonnage is my final goal and I think ultrasound can give it to me. By measuring acid concentration and flow on the actual product line, we will be able to calculate the mass flow rates real time and know exactly what we’re producing all the time. Right now we only have a flow meter on the product line, but since it doesn’t measure density or concentration compensation, we have to rely on the strapping of our sulfur tanks at the end of the production day. We strap the tanks to figure how much inventory changed and compare it to what we measured. So if we think we produced 7,500 tons of product and used 2,500 tons of sulfur, but our strapping results show we only used 2,400 tons, then we adjust our total production to 7,200 tons. This creates noise in our production and monitoring data, so if we can use this online system it will allow us to know exactly what we’re producing all the time in real time.

By keeping up with the latest technology, we have not only achieved its primary goal of increased safety, but also greatly reduced downtime and will soon gain real time control of production data.

Mark Hagemo is the Sulfuric Acid Superintendent at the Mosaic plant in Riverview, Florida.

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