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Real-time monitoring of dissolution processes

What fixed time intervals can only estimate, LiquiSonic measures® - continuously, inline, and in real time.

The challenge

For example, in stirred reactors, preparation tanks, or batch vessels, the progress of dissolution cannot be observed from the outside. That is why most systems operate by default with fixed timers, because it cannot be determined with certainty when dissolution is complete and the batch is actually finished. This conservative buffer costs real money. Every extra minute ties up reactor capacity, consumes energy, and delays the next batch. If the batch is stopped too early, undissolved solids remain. This also has direct consequences for product quality and downstream processes. The only alternative, manual lab sampling, is labor-intensive and delivers the result far too late.

Our solution

The LiquiSonic® measurement system is installed directly in the vessel and continuously measures the speed of sound in the liquid in the tank. This changes with concentration: the more solid dissolves, the higher the measured value. As long as dissolution continues, the measured speed of sound increases steadily. The process temperature measured at the same time is used for compensation so that the measured value remains meaningful at all times. As soon as all solids are completely dissolved, it stabilizes at a constant plateau at the target value.

In addition, the attenuation of the ultrasonic signal is monitored. It provides additional indications of undissolved components, particles, or gas bubbles in the medium and thus supports the assessment of whether the process is proceeding homogeneously and stably.

The process step by step

The LiquiSonic® measurement system accompanies the batch throughout the entire process. As soon as the solid is added and the mixing process starts, measurement begins immediately.  In this first phase, the speed of sound is still unstable because undissolved particles or gas bubbles can influence the measurement. The simultaneous attenuation measurement makes these visible and thus supports the evaluation of the dissolution progress.  As dissolution progresses, the speed of sound in the liquid steadily increases. The sensor continuously detects this increase and responds immediately to every change in the process. The more solid goes into solution, the more smoothly and specifically the measured value shifts toward the target value.

Shortly before the target value, the curve flattens out. This is a clear sign that most of the solid has now dissolved and the dissolution process is approaching its end. The sensor detects this slowdown and prepares for endpoint detection. As soon as the speed of sound remains stable at the target value, the endpoint has been objectively reached. The solution is completely homogeneous, and all solids are dissolved. The system can automatically release the batch, so no intervention, no lab sample, and no safety buffer are needed.

Advantages

Using LiquiSonic® makes every dissolution process measurable, efficient, and reliable:

  • Shorter cycle times: Batches end exactly when they are finished, not when a predefined timer expires
  • More capacity: Vessels are released earlier for the next batch
  • Lower energy consumption: No unnecessary continued mixing or heating after the endpoint
  • Assured product quality: Complete dissolution is confirmed objectively
  • No manual sampling: Process and release can be fully automated


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Frequently asked questions

How can dissolution processes be monitored?

LiquiSonic® continuously measures the speed of sound in the process liquid. Since the speed of sound changes directly with the concentration of dissolved substances, the progress of dissolution can be tracked in real time. At the same time, the attenuation of the signal and the process temperature are recorded and used for compensation, so the measured value remains reliable even with a high proportion of gas bubbles or fluctuating temperatures.

Which solids and media is the system suitable for?

The measurement principle is substance-independent and works wherever the speed of sound of the liquid changes with the progress of dissolution. Typical applications include salts such as potassium sulfate and sodium chloride, sugar, polymers, caustic and acid solutions, as well as CIP and tank rinsing processes. For certain media, SensoTech recommends an application-specific calibration.

How reliable is the measurement system in practice?

The endpoint is detected based on a stable plateau of the speed of sound at the defined target value. It is an objective, reproducible signal that is independent of operator judgment or lab results. SensoTech adapts the detection parameters to the specific application and process to ensure the earliest possible reliable endpoint signal and tight, repeatable batch cycles.

Does the measurement also work with slowly dissolving solids?

LiquiSonic® continuously tracks the dissolution profile and responds immediately to changes in the process. Even with slowly dissolving solids, the system reliably detects the endpoint of the dissolution process because it does not wait for a fixed target value, but instead uses stabilization of the measured value as the criterion. Application-specific adjustments to the detection logic are possible.

What happens with fluctuating input quantities?

LiquiSonic® captures the progress of dissolution in real time and reflects process changes without delay. Even with poorly soluble solids or fluctuating input quantities, the system reliably determines the completion of the dissolution process. The basis for this is not only reaching a rigid setpoint, but the stabilization of the measured value. The endpoint detection logic can be adapted to the respective application.

Can LiquiSonic be used in multiple vessels at the same time?

Yes, multiple sensors can be connected to a central evaluation unit via a shared sensor bus with a cable length of up to 1000 m. This enables the parallel monitoring of multiple reactors or vessels from a single system.

How can the measurement system be integrated into an existing plant control system?

The LiquiSonic® Measuring system supports common industry standards for signal output and can be easily integrated into existing process control systems. The end point can be transferred directly to the system control as a digital or analog signal, for example to fully automate batch release.

How high is the maintenance effort during ongoing operation?

LiquiSonic® works without moving parts, without consumables and without regular calibration in normal operation. The sensor is designed for continuous operation and requires no routine intervention, which can significantly reduce unplanned downtime and operating costs.

What are the disadvantages of a timer-based process in resolution processes?

Timer-based process control is based on conservative estimates, not on the actual process status. This systematically leads to excessively long batch times, unnecessary energy consumption and tied up reactor capacity. If the timer is set too short, incomplete resolution will have negative quality consequences. Continuous measurement of the speed of sound replaces the timer with an objective, real-time endpoint signal, allowing each batch to end exactly when dissolution is actually complete.

What alternatives are there to manual sampling in dissolution processes?

Manual laboratory samples always provide a delayed picture of the process status. The sample is taken, analyzed and evaluated while the process continues. In addition, manual sampling is labor-intensive, error-prone and associated with safety risks when using aggressive or temperature-sensitive media. LiquiSonic® measures the dissolution state directly in the container several times per second and completely eliminates the need for manual intervention for endpoint control.

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