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NMP recovery:
Continuously monitor water content in the process

LiquiSonic® Ultrasonic sensors determine the water content in the NMP/water mixture directly in the pipeline or column, inline and without sampling.

Whether in the condensate, in the distillation column, or in the processed recovery product: the measured value is continuously available, even if isolated gas bubbles occur in the process.

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Continuous instead of spot-check sampling

One current water content value per second instead of one sample per shift

Without wear parts

No moving parts, no seals, no optical window to clean

Gas bubbles in view

Detects gas bubbles in the condensate to a certain extent as an additional signal

Available for hazardous areas

Versions for explosion-hazard plant sections available

The water content in an NMP/water mixture can be measured with a LiquiSonic® ultrasonic sensor directly in the pipeline or column, without needing to take a sample. The sensor determines the temperature-compensated speed of sound of the liquid; since this has a fixed relationship with the ratio of water to N‑methyl‑2‑pyrrolidone (NMP), the current water content is continuously derived from it. The method is suitable for condensate, distillation, and recovery streams in NMP recovery systems for cathode manufacturing plants and provides a new measured value every second, regardless of the medium's turbidity, color, or conductivity.

Where control becomes difficult in practice

N‑methyl‑2‑pyrrolidone (NMP) reliably dissolves the PVDF binder in cathode slurry and can be removed again without residue during drying. That is why it is the solvent of choice in many coating processes in cathode manufacturing. When the coated foil is dried, the NMP evaporates and is carried away with the oven exhaust air. Because NMP is expensive and exhaust air limits for solvent vapors are strict, it is condensed from the exhaust air in recovery systems, separated from the co-condensed water, and returned to slurry production. How much NMP is present at what time in the condensate, in the distillation column, and in the processed product determines yield, energy consumption, and the quality of the next slurry batches.

Typical challenges

  • Fluctuating NMP/water ratios in the condensate depending on oven load
  • Distillation column must be continuously adjusted to changing feed
  • Residual moisture in the processed NMP affects the next slurry batch
  • Startup and shutdown phases with highly fluctuating composition
  • Isolated gas bubbles due to boiling or vacuum operation of the column

Consequences of insufficient monitoring

  • Distillation out of balance, higher energy consumption
  • NMP that is too wet leads to inconsistent slurry viscosity
  • Unnecessary consumption of fresh NMP at a low recovery rate
  • Delayed response because lab results are only available later
  • Unplanned downtime when readjusting the column

Why common methods fail

  • Lab sample with gas chromatography or titration: meaningful, but hours behind
  • Karl Fischer titration determines water content precisely, but only as a lab sample, not continuously in the process
  • Refractometers react sensitively to contamination and temperature fluctuations
  • Density measurement alone does not provide a clear statement about water content in NMP/water
  • Conductivity says little about water content, since NMP itself is hardly conductive

How LiquiSonic measures® water content in NMP inline

The sensor is installed directly in the process, in the condensate stream, in the column, or in the return line and determines the transit time of an ultrasonic signal through the liquid. The speed of sound is derived from the transit time and the known measuring path, and from this, with temperature compensation, the water content in the NMP/water mixture.

Independent of color, turbidity, and conductivity
The speed of sound is a physical property of the mixture, not an optical or electrical substitute variable that could be distorted by discoloration in the condensate.

Integrated temperature compensation
Each sensor also measures the process temperature with high precision and directly compensates for its influence—important with changing operating points of the column.

Gas bubbles detectable to a certain extent
Isolated gas bubbles, for example due to boiling or vacuum operation, change the shape of the received signal. The sensor outputs this as an additional signal; at very high gas content, this method also reaches its limits.

Calibratable to accompanying components
If additional dissolved substances remain in the mixture, such as small binder residues, within known narrow limits, the calibration can be adjusted accordingly.

How the measurement appears during operation

The LiquiSonic® controller displays the current measured value directly at the process, without detouring through a control system.

Water content in plain text
The main value is shown large and easy to read in the center, including the unit.

Permissible range at a glance
The bars on the left edge show whether the value is within the limits defined for the respective process point.

External alarm
If the water content leaves the permissible range, this can be sent directly to a digital output or to the control system.

How measurement works in the NMP recovery process

Ultrasonic transit-time measurement

Transmitter and receiver are positioned at a fixed distance in the sensor head. The speed of sound is derived from the signal transit time and evaluated with temperature compensation—an absolute, drift-free measured value that clearly depends on the water content in NMP. If the mixture contains additional components within known narrow limits, this can be taken into account through application-specific calibration.

Gas bubbles as an interfering factor

Gas bubbles not only change the speed of sound, but also the entire shape of the received signal. The sensor evaluates this and outputs it as an additional signal—making it possible, to a certain extent, to detect that gas bubbles are present in the condensate or in the column, independently of the actual water content measurement.

Economic benefits

Less fresh solvent

✔ - Continuous visibility into the separation performance of the column

✔ - Less NMP loss through residual moisture in wastewater

✔ - Traceable metrics for the recovery rate

More stable slurry quality

✔ - Defined residual moisture in the recovered NMP

✔ - Less batch variation in subsequent coating

✔ - Early signal when the processing shifts

Less effort in the lab

✔ - Fewer manual samples and GC analyses in daily operation

✔ - Up-to-the-second values instead of waiting for lab results

✔ - Laboratory capacity free for release measurements and special cases

Individual adjustments

Installation options

  • Sensor lengths from 92 mm to 3,000 mm
  • Connections for pipeline, column or bypass
  • Adapter for small and large cable cross-sections
  • Versions available for use in hazardous areas

Material options
NMP and water do not pose any special requirements for standard stainless steel. For more aggressive accompanying substances or abrasive residues, Hastelloy and coatings such as PFA, ETFE or Halar are available.

Mixtures with accompanying components
If other dissolved or entrained substances in the mixture remain within known, narrow limits, the calibration of the sensor can be adjusted accordingly without it becoming an independent second measuring task.

OEM and special designs
Even for smaller quantities, we create customer-specific designs, connections and interfaces for recovery systems and their component suppliers. Talk to us about it.

Quick overview

Measuring ranges, accuracies, temperature and pressure levels as well as materials depend on the design and application. We design the sensor together with you and, where relevant, with the system manufacturer of your recovery unit and provide the reliable values ​​for your process.

  • Measuring principle: Ultrasonic, temperature compensated, inline
  • Measured variables: Water content in NMP, temperature, disturbance detection (e.g. gas bubbles)
  • Installation: Piping, column/sump or bypass, sensor length and process connection according to application
  • Interfaces: Analogue outputs, digital outputs for alarms, Modbus, Profibus, Profinet, Ethernet/IP
  • Maintenance: Maintenance-free, one-time factory calibration

Practical examples and use cases

Condensate and column monitoring

Continuous visibility of the water content in the feed to distillation as the basis for stable column operation.

Release of recovered NMP

Checking the residual moisture in the processed solvent before it goes back into the slurry mixture.

Commissioning of new recovery units

EPCs and plant operators gain an immediate, reliable overview of separation performance when ramping up new recovery systems.

35 years of experience in inline concentration measurement

SensoTech has been developing measurement technology for liquids in the process industry since 1990. Our systems operate worldwide under demanding conditions, from chemicals and pharmaceuticals to battery manufacturing.

35+

Years of experience

50+

Countries worldwide

ISO 9001

Quality certified

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Frequently asked questions

How can I tell that our NMP recovery is no longer running optimally?

Often only late: The column is still separating, but with higher energy consumption, or the recovered NMP is wetter than assumed, which only becomes noticeable in the next slurry batch. Continuous measurement in the condensate and at the column outlet reveals such shifts while there is still time to make adjustments.

What distinguishes ultrasound measurement from gas chromatography or titration?

GC analysis and titration provide precise individual values in the lab, but with a delay of hours. LiquiSonic® measures directly in the process and delivers a new value every second without sampling - as a continuous supplement, not a replacement for occasional lab checks.

How is the speed of sound related to the water content in NMP?

The speed of sound in the NMP/water mixture changes with its composition. Therefore, the water content in NMP can be clearly determined from the temperature-compensated measurement of the speed of sound.

Is density or conductivity measurement not sufficient?

Both variables are also related to composition, but they react sensitively to other influences: density, for example, to gas bubbles, and conductivity hardly to water content since NMP itself is practically non-conductive. The speed of sound provides a more robust basis for this.

Can the sensor also detect gas bubbles in the condensate or in the column?

Yes, to a certain extent. Gas bubbles change the shape of the received ultrasound signal, and the sensor evaluates this as an additional signal. However, if the gas content in the medium is very high, this method also reaches its limits.

Does the measurement also work if the mixture contains additional components?

Yes, as long as their proportion remains within known, narrow limits. We then tailor the calibration to your specific mixture, for example if small amounts of binder residues or degradation products are carried along.

Is the solution also available for use in hazardous areas?

Yes. For plant sections with explosion protection requirements, we offer appropriately certified sensor versions that cover the usual requirements of recovery systems.

How quickly is a measured value available?

In real time, with a new value every second. This is sufficient to detect deviations in condensate, column, or product stream while there is still time to intervene.

Will the sensor fit into our pipeline or column?

Generally, yes. Standard process connections and flanges are available, insertion lengths from 92 mm to 3,000 mm, as well as adapters for small and large cross-sections. For special cases, we manufacture according to your requirements.

What maintenance is required?

None during operation. There are no moving parts, no seals, and no optical window that could become dirty. Recalibration is normally not necessary.

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