Phosphate Plant Boosts Safety and Production

Ultrasonic metering minimizes acid leaks and allows real-time control.

By Chris Hagemo, The Mosaic Company

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Mosaic is the world's leading producer of concentrated phosphate, a key ingredient in most fertilizers. We operate in every phase of phosphate development, from mining rock phosphate to making phosphate fertilizer. Production of fertilizer requires sulfuric acid. Leaks caused by the corrosive acid forced sporadic shutdowns at our Riverview, Fla., plant.
 
To create phosphate suitable for fertilizer the phosphate rock is crushed, mixed with water and then reacted with sulfuric acid to produce phosphoric acid. Finally, reacting the phosphoric acid with ammonia yields ammonium phosphate fertilizer.

The sulfuric acid is made on site from molten sulfur and water. It's critical to carefully monitor both acid flow and concentration. We were using temperature-compensated conductivity meters, also called titrimeters, on a slipstream of acid from the main recirculation flow to measure concentration. These meters provide accurate information when they're working properly — but they're intrusive, coming in direct contact with hot sulfuric acid, and frequently failed, resulting in leaks. These leaks, rather than the instrument failures, were our biggest concern because of their safety and environmental impacts.

Eliminating or at least decreasing how often we have to send flow through these smaller lines would substantially reduce the risk of leaks.

About two and a half years ago, I mentioned the problem to the account executive from Solares Controls, a manufacturer's representative and distributor that we'd been working with for years. He recommended ultrasonic meters. We already had installed several ultrasonic units from Flexim on our water and gypsum lines strictly to measure flow and were pleased with their accuracy. He told me Flexim also made a meter, called the PIOX S, that measured concentration and temperature as well as flow. Because ultrasonic meters are non-intrusive, I was immediately interested.

THE TECHNOLOGY
Most ultrasonic flow meters use a technique called transit time difference. This exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium; the signal moves slower against the flow than with it.

A meter sends pulses through the medium, one in the flow direction and one against it. The transducers alternate as ultrasonic emitters and receivers. The meter measures transit time difference and determines the average flow velocity of the medium. Because ultrasonic signals propagate through solids, you can mount the meter directly on a pipe and measure flow non-invasively.

The Flexim PIOX S uses the data from the ultrasound transmission to perform concentration analysis and to calculate the concentration-compensated mass flow rate. The instrument also is useful for supervising crystallization or phase-separation processes for proper product identification and for determining density, yield rate, Brix or other customer-specific measurement values.

For the sulfuric acid service, I'm more interested in concentration than flow.

METERS PROVE THEMSELVES
I decided to take the cautious route — installing one meter and monitoring its accuracy for a year. It proved satisfactory, prompting me to think we could 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 the meters on all of our interpass acid lines. We now have 20 Flexim PIOX ultrasound meters performing well and sending data back in real time. They have improved safety, accuracy and production.  

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

Many of the ultrasound meters back up our titrimeters but a few serve as primary measurement devices. As they continue to prove their value I plan to phase them in as primaries to eliminate intrusive metering.

Accurate measurement of product tonnage is my final goal; I think ultrasound can give it to me. By measuring acid concentration and flow on the actual product line, we'll be able to calculate the mass flow rates in real time and know exactly what we're producing all the time.

Right now, we only have a flow meter on the product line. Because it doesn't measure density or provide 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 has changed to correct what we measured. 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. We expect the online system will allow us to know exactly what we're producing all the time in real time.

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


CHRIS HAGEMO is the sulfuric acid superintendent at the Riverside, Fla., plant of The Mosaic Co. E-mail him at chris.hagemo@mosaicco.com.

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