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,” predicting 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 the level 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. Inch counts may be taken as often as 10 times a day. The problem is, before taking a manual inch count, operators had to stop the reactor and wait for the contents to cool. This resulted in a great deal of lost production time.

Person on a mission

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 a global technology leader in the development, manufacture and distribution of accurate, reliable and cost-effective measuring instruments for the process industries. “I had used KROHNE equipment at another chemical facility, and knew their equipment,” Yarborough says.

Michael Barber, Southeast district sales manager, recommended the OPTIWAVE 7300 C, a non-contact frequency modulated continuous-wave (FMCW) radar level meter. According to Barber, “The unit 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.”

The device offers four different types of screen displays depending upon user preference: alphanumeric, alphanumeric with tank illustration, alphanumeric with bar graph and signal screen. The FMCW meter also features a device type manager (DTM) — a device driver that makes functionality available independent of the fieldbus protocol and also provides a graphical user interface to make process setup and analysis extremely easy.

Post-trial decision making

After conducting a two-week trial demonstration, Yarborough purchased the OPTIWAVE 7300 and never looked back. Installation and set-up was simple, since all that needs to be done is fitting the gauge to the tank, wiring it and turning it on. 

“The supplier 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 says. “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.”
Temperature in the reactor reaches about 105 C. Yarborough notes that he has seen high ambient temperatures and huge temperature swings and the device still functions, not getting lost in the vapors.

According to Yarborough, one of the challenges in finding the right level measurement device was that the material’s dielectric is very low, i.e., less than two. Such low dielectric provides weak reflection and can be difficult to measure with microwave energy. 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.

Barber notes that his instrument’s advanced design offers a maximum measuring range of 131 feet. 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 device allow the detection of the smallest level changes and clearer location of the product’s true surface,” says Barber. “In addition, objects such as struts, inlets and ladders, and even agitated surface or foam, have little effect on signal strength.”

Final words

Long antenna versions of the instrument can be extended to suit any 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.

The company started with three OPTIWAVE 7300 C level meters and intends to gradually install them on all its remaining reactors. “The physical size works well with our operations,” Yarborough says. “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. 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.”