A North Carolina-based specialty chemical manufacturer, a major producer of insect repellent, was looking for a better way to measure the liquid level in its glass-lined agitated reactor. The company uses a number of complex technologies to manufacture sebacates, adipates, isophthalates, catalysts, alkyds, and other natural and renewable chemistries based on castor and citrates.

The chemical manufacturer had always relied on the low-tech “inch count” method, which means an operator actually inserts a tape measure into the hatch and measures down to the liquid level to calculate how many gallons are in the reactor.

The information is used to calculate the “drying rate” to predict when the processing is finished and the product is ready for post-processing. Operators look at starting and ending gallons to calculate the percentage of moisture, which is important for product quality. If they start at a certain inch level and apply heat to the vessel, they measure how fast it is dropping to determine how many gallons have dried.

The manual inch count is also used to “charge the heel,” which refers to dispensing starting chemicals for the next batch. Based on the amount of heel material in the reactor, operators can calculate how much moisture and other chemicals must be added for the next batch.

The information is critical to the manufacturing process and inch counts may be taken as many as 10 times a day. The problem is, before taking a manual inch count, operators must stop the reactor and wait for the contents to cool. This resulted in a great deal of lost production time.

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OPTIWAVE 7300C FMCW radar shown on tank top. Note nozzle installation close to tank side wall.

In Search of an Easier Level Measurement Solution
The company’s engineering manager, Todd Yarborough, went looking for a less time consuming method of getting the vital information. He was also seeking a way to minimize worker exposure to chemical vapors used in the production process.

The engineer sought out KROHNE, a manufacturer of measuring instruments for the process industries, because he had experience with their equipment at another chemical facility.

Michael Barber, KROHNE’s Southeast district sales manager, recommended the OPTIWAVE 7300 C, a non-contact frequency modulated continuous-wave (FMCW) radar level meter. According to Barber, “The OPTIWAVE 7300C model is designed for distance, level, volume, and mass measurement of liquids, pastes and slurries. It gives a more stable measurement than pulse radar and is well suited to agitated process conditions. It can operate at very low and very high process temperatures as long as the process connection temperature limits are observed.”

After conducting a two-week trial demonstration, Yarborough purchased the OPTIWAVE 7300. Installation and set-up of the OPTIWAVE 7300 C only required fitting the gauge to the tank, wiring it, and turning it on.
“KROHNE configured the unit in 10 minutes and the device immediately began continuously measuring how many inches and gallons were in the vessel without stopping or operator interaction,” Yarborough said. “We got immediate feedback from operators that they loved it, because it enabled them to perform their duties without opening the hatch. In addition, using the meter to eliminate the manual inch count reduced our cycle time considerably.”

The temperature trend in the reactor was about 105 C for this chemical reaction. Yarborough said that he has seen high ambient temperatures and huge temperature swings and the device still functions, not getting lost in the vapors.

Process reactor tanks require frequent measurements. Radar level improved accuracy, operator safety and production throughput.

Low Dielectric Presented Level Measurement Challenges
According to Yarborough, one of the challenges of finding the right level measurement device was that the material’s dielectric is very low, less than 2. Such low dielectric provides weak reflection and can be difficult to measure with microwave energy, so he was concerned whether the device could maintain a strong signal. The equipment demonstration proved that the device could track levels even with the low dielectric.

The OPTIWAVE 7300 C offers a maximum measuring range of 131 feet (40 meters). This enables it to operate with a larger bandwidth, ensuring sharper resolution and resulting in measurements that are more accurate as well as repeatable. “The higher signal dynamics of the OPTIWAVE 7300 C allow the detection of the smallest level changes and clearer location of the product’s true surface,” Barber said. “In addition, objects such as struts, inlets, and ladders, and even agitated surface or foam, have little effect on signal strength.”  

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The OPTIWAVE offers long antenna versions that can be extended to suit different nozzle length. It can be equipped with a drop antenna for corrosive liquids (with optional PTFE/PP flange plate) or where product build-up is likely to occur. A sealed drop antenna extension option is available for pressurized tanks.

According to Barber, “In this application, we used the smallest antenna we have, 1.5-inches, in order to facilitate the installation. However, the smaller the antenna, the larger the beam angle, resulting in wide dispersion of the signal over distance. Signal intensity is the weakest with this antenna, which does add to the measurement challenge, yet this is overcome by the tremendous signal processing of the Optiwave electronics.”

The company started with three of the OPTIWAVE 7300 C level meters and intends to gradually install them on all its remaining reactors. “The physical size works well with our operations,” said Yarborough. “Also, the required antenna was an easy installation for us, because the existing nozzle and antenna size fit into an existing nozzle on the reactor. Actually, one reason we went to higher frequency radar was to make it compatible with the existing nozzle. Another nice feature is that I have all our instrument configurations on file, so it will be easy to connect new instruments and download configurations as they come on line.”