Five years ago, we published an article on in-line vs. off-line moisture measurement. At that time we debated whether these measurement systems should be called on-line or in-line. We chose to use the term "on-line". Since then, however, the world of the internet became known as "online". So, to avoid confusion, we changed our view and now refer to direct measurement as "in-line" moisture measurement.
In the five years since we wrote that article, traditional methods of capacitance, radio frequency and near infrared matured. Newer methods such microwave, nuclear magnetic resonance, and tuned laser diode developed to take advantage of the evolving technology. Some of these new techniques made instantaneous in-line moisture measurement attainable in products where in-line moisture determination was previously impossible.
In addition to moisture, the range of in-line solutions have expanded to include parameters such as density, viscosity, chlorine and protein.
Direct vs. Indirect Measurement of Moisture
Direct methods of moisture determination, such as loss-on-drying, remove water from a sample and enumerate the result. To get results from these direct methods takes anywhere from a few minutes to several hours. These methods are not easily adapted to in-line applications.
A common characteristic of in-line compliant methods is that results are virtually instantaneous. However, these techniques measure parameters such as capacitance and wave lengths. These are indirect measures of moisture levels. To get to a measurement in moisture terms, these devices must be calibrated against samples of known moisture.
Because of rapid measurement response, the promise of testing moisture in-line is like a siren’s song. Many believe that all you do is plop the device on your production line and voilá - you have continuous, instantaneous moisture readings and perfect process control. In reality, it’s not quite that simple.
To figure out if in-line solutions makes sense for your application, it is imperative to understand the following:
- the characteristics of the process,
- the reasons for moisture testing, and
- the uses made of the test results.
Grab Sample Testing
If you are using grab samples for moisture analysis, a look at 1) the control potential of the process and 2) the purpose of the measurements. This is the starting point of evaluating the potential benefits of changing to an in-line system.
Grab samples are usually taken at the end of a batch, periodically at fixed locations in a process, when receiving incoming raw materials, or for QC checks on the finished product. Grab sample moisture testing is mostly done with direct measurement devices such as a loss-on-drying moisture balance or a Karl Fischer titrator. The test results serve one or more of the following purposes:
- Historical recording of the process.
- Reporting to regulatory authority.
- Go-No-Go determinations for end-product or raw material acceptance.
- Grading of end-product.
- Defining specifications for delivery reports.
- Diagnosing potential process changes for the next batch or production run.
- Identifying on-the-fly adjustments during the process.
If the results are used for on-the-fly or real-time adjustments to the process, the grab sample procedure is a strong candidate to be replaced by an in-line system.
If the main purpose of the testing falls into the other categories, the pay-off of switching to an in-line system may be marginal.
A continuous process in which moisture levels can be modified by upstream actions such as changes in temperature, addition of ingredients, changes in pressure and the like, is a prime candidate to benefit from an in-line solution.
Batch processes, in which changes in the final result can be affected during the time of the batch are also candidates to show a strong in-line return on investment (ROI).
The most “bang for the buck” happens when you can control the moisture in the process through an automatic, feed-back-loop control system. A feed-back-loop changes the process based on the in-line measurement.
If the collection and analysis of real-time data is the only purpose of the testing, but the analysis of the data has potential to define substantive future process changes, an in-line solution may have a high potential ROI and suggest a detailed inquiry.
In some cases, an in-line solution for reporting only may pay-off because of reduced cost of testing, supervisor expense or reduction of human error. This is a lower priority area for analysis.
Examples of Direct Payouts
A mining company wanted to reduce the cost of its drying operation. Material was delivered to a dryer from an outside conveyor. Moisture in the mined material varied depending on where it had been stored, environmental conditions and condensation.
Historically, the dryer had been set to deal with maximum drying temperature and dwell time. The company did not have a way to detect moisture variation in real-time.
A moisture measuring system was installed on the conveyor to get real-time monitoring. Based on the moisture levels detected, the dryer controls were automatically changed to control temperature and dwell time. This measurement and control system produced significant power cost saving and a quick system payout.
Food Pellet Operation
A second example is a plant that produced pelletized products. If process material exceed a specific moisture level, the pelleting equipment was fouled. This caused the process line to be shut down for several hours. At the other extreme, material that was too dry would not meet the specifications for the final product. This resulted in scrap product and often caused excessive wear on the pelletizers.
This process starts with the formulation of ingredients in a mixer. The mixing process was enhanced by the addition of steam. The amount of steam applied is directly related to the moisture of the material in the process.
The plant had been using grab sample testing. This made process optimization difficult. Achieving acceptable results depended heavily on the expertise of the operators.
An in-line moisture instrument was installed in the process line, downstream of the mixer. Measurements from this were used to automatically adjust the steam. This maintained the required amount of moisture and nearly eliminated the need for maintenance of the pelletizers. In addition, the system reduced the time for to start-up and automatically optimized the line when different products were produced.
Rejection of end-product because of wrong moisture levels was vastly reduced.
Evaluate the Process
Both examples illustrate the potential benefits of in-line systems. Processes in which real-time measurements can be used in a feed-back-loop to control variables are prime candidates for an in-line system. Elimination of guesswork regarding process adjustments, fewer process shut downs, optimization of process inputs, such as power, raw material and processing time and significant lowering of QC rejects can lead to an attractive ROI.
In-line solutions have initial costs of 10, 20, even 30 times more than grab sample techniques. Even with this high initial cost, operations can be improved to levels that produce rapid payouts.
However, justification of in-line measurement systems for the sole purpose of collecting measurement data and incorporating it into data systems, is a more complex analysis. Sometimes cost saving from reductions in testing staff, supervision requirements, and problems resulting from human error, and the more subjective consequences of research use of the data, can produce a positive ROI. These analyses are often inconclusive.
In-line moisture systems have the potential to impact the results of an operation. A process with the following characteristics is an initial candidate for a cost effective result from an in-line system:
- The process can be changed and controlled on the basis of instantaneous moisture measurement.
- Significant cost saving, in terms of energy, spoilage, down time, QC rejects or customer complaints, can be realized through real-time control of moisture.
- The value of collecting and digitally recording continuous data should be more than mere convenience.
- Cost saving only in the collection of moisture data may be difficult to justify.
In-line systems can be more than a Siren’s Song. For processes that can be closely controlled using real-time measurements, in-line systems in feed-back-loops have high promise for significant improvement in finished product quality, energy saving, reduction of spoilage and down time. They can produce a high ROI.
In conclusion, I urge a careful analysis of your process against these criteria.
Every time I think I am on to a good clear answer for a measurement issue, such as “Do In-Line Measurement Systems Really Produce?” I run into the caveat “it depends”.
Hope this discourse has been helpful. Please share it with associates who may be able to use it.
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P.P.S. Learn more about NIR in-line moisture measurement. Click on this image: