Flow Challenge - Measuring the flow of chemicals can prove very difficult if the underlying manufacturing process is not handled or controlled properly. The often corrosive nature of such liquids challenges conventional measurement methods based on insert-type sensors, which may ultimately jeopardize the quality of the end-product. This was the challenge facing a chemical company in China when looking for a number of flow meter technologies to be used in one of its Chinese manufacturing plants slated for expansion.

The upgrade that the chemical company planned would affect a facility that manufactured glyphosate at an annual production rate of 20,000 tons. With the expansion, another 50,000 tons would be added to the facility's output, making it the largest glyphosate manufacturing facility in Asia. Glyphosate it is one of the most widely used herbicides and is commonly used to kill weeds and plants in industrial as well as home use. Although not the case at this facility, glyphosate can also be used to manufacture water treatment chemicals, flame retardants, and lube oil and paint additives.

Precision Essential to Product Quality

One major step in glyphosate manufacturing is to make dimethyl phosphate in which phosphorus trichloride (PCl3) is used. As part of this process, PCl3 and methanol are pumped at a set ratio through two separate lines to a reactor where a chemical reaction under vacuum conditions takes place. This process, which is called esterification, leaves behind only dimethyl phosphate.

The quality of the final glyphosate product is directly related to the precision of the injection ratio between the PCl3 and methanol. One of the major problems with PCl₃, however, is its aggressive nature, which makes it challenging to control:

• _PCl3 is highly flammable and generates toxic hydrochloride fumes when burning
• When in contact with air, PCl₃ becomes very corrosive
• In an uncontrolled or poorly controlled environment, PCl₃ crystallizes easily
• PCl₃ becomes explosive when in contact with water or acid

Due to these chemical properties of PCl₃, it had been difficult for the chemical company to find a flow measurement technology capable of obtaining the desired results in regards to precision and reliability while maintaining the required degree of safety in the production process.
Manual flow control leads to high raw material consumption

In the past, mechanical insertion-type flow meters had been used whose gears were easily corroded by the chemical. This would result in crystal build-up with poor or failing meter performance as a consequence. To remedy the situation, the meters had to be frequently removed for cleaning, forcing the manufacturer to rely on manual flow control. That, in turn, caused much higher raw material consumption compared to what could be obtained by automating the process and also created fluctuations in product quality.

Although the chemical company had considered using clamp-on ultrasonic flow meters, which would normally be considered an optimal solution for such an application due to the non-intrusive installation method, only narrow beam ultrasonic flow meters had been used to conduct tests. The results were not promising, primarily because of the low flow rates of 0.1-0.15 m/s (0.3-0.5 ft/s), small diameter pipes in the DN50 (2 in.) range, and the presence of aeration in the chemical. All of these factors prevented the narrow beam clamp-on ultrasonic flow meters from performing optimally, forcing the company to discard the technology as a viable option.

Testing Before Committing

For the facility expansion project however, the clamp-on ultrasonic flow measurement technology was given a second chance. Siemens offered to conduct a test that would demonstrate how precise and reliable measurement could be obtained while allowing for the esterification process to be fully automated. The means to achieve this was the Sitrans FUS1010 clamp-on ultrasonic flow meter.

The dual channel meter was tested for one month under various conditions. With reference flow volumes ranging from 186 to 744 m³/m (6,570 to 26,275 ft³), the Sitrans FUS1010 was able to measure with an accuracy of up to 0.44 percent of flow rate. The chemical company was very satisfied with the results, especially when taking the low flow rates, small pipes and aeration conditions of the liquid into consideration.

Wide Beam Vs. Narrow Beam Measurement

What makes the wide beam technology so suitable for chemical applications is that the resonant frequency of the pipe wall is utilized to achieve a strong ultrasonic signal. Upon installation, the transducers broadcast signals with varying frequencies. The aim is to find the frequency that best matches the pipe wall. When found, the signal is transmitted into the flowing media with the wall of the pipe acting as a waveguide. This method allows for a low transmit voltage of approximately +/- 15 volts and produces a focused, coherent signal that covers a large axial area (fig. 1).

Because this technology makes use of a wide beam it is much more resilient to aeration and suspended solids that may exist in the fluid. It would simply require a much higher concentration of particles or higher percentage of aeration to disturb the signal to a point where measurement will be impacted.
Typically, this does not occur even with fluids containing particles in the ten to 12% range.

The narrow beam method, on the other hand, which is also known as shear mode, generates a high but narrow energy acoustic signal, typically 1 MHz, which is received by the opposing transducer (fig. 2). The elevated transmit voltage combined with the single transmit frequency often produces a high level of noise. This is ultimately present on the receive signal, producing a poor signal to noise ratio, which will diminish performance.

Reaping The Benefits

For the chemical company, the determining factor was the clamp-on ultrasonic flow meter's ability to deliver on the precision and reliability parameters through its use of the wide beam transit time technology.

Accuracy, however, was not the only important benefit of the Sitrans FUS1010. With a dual channel flow meter they could measure both PCl3 and methanol using only one flow transmitter, ensuring additional benefits. First and foremost, it reduces the total investment at each measurement point because only one meter is needed to measure on two pipes. It also maintains the system's integrity since the same supplier's flow meter delivers the complete solution. This facilitates the elimination of any errors and increases measurement repeatability and accuracy.

Another benefit that the chemical company was able to obtain using the Sitrans FUS1010 clamp-on ultrasonic flow meter was the measurement of the aeration percentage of the chemical. This information was used to identify process conditions in real-time, which aided in the selection of the optimal location - with less aeration - for the meter installation, and also the optimization of the process. Additionally, the clamp-on ultrasonic meter could be installed without opening the pipes or stopping the process, eliminating costly manufacturing down-time and subsequent system re-optimization.

Optimized Manufacturing Process

In the glyphosate manufacturing process, a varying ratio of PCl3 and methanol will yield a differing end-product quality. By installing several clamp-on ultrasonic wide beam flow meters, the Chinese chemical manufacturer was able to improve the accuracy of the raw material injection ratio into the reactor, optimizing the manufacturing process and the product quality while reducing raw material consumption. All in all, the Siemens solution gave the chemical company an increased economic return and overall satisfaction.