In industries that produce powders and bulk solids, controlling moisture content can substantially impact the production costs and yields, material handling, purchase price, shipping costs, and, perhaps most importantly, the quality of the final product.

When processing dry bulk solids such as food ingredients, chemicals, pharmaceuticals, pigments, minerals, etc., the amount of moisture in the product can have a wide range of effects. A product’s quality, as well as transactions based on weight, can be adversely affected by improper moisture contents. Furthermore, the satisfaction of legal requirements that are present in many industries is a determining factor influencing a company to measure moisture content.

However, until recently conducting frequent moisture content tests throughout the process or in the field has been difficult. In many cases, the primary barrier has been the expertise and time required to conduct such tests. Often sophisticated moisture measurement devices must be operated by trained personnel that can properly calibrate the equipment. Many also require meticulous sample preparation and disposal.

Fortunately, new inline form factors are now available that allow unattended measurement on a process where the product is not visible. This allows the optimization of mixing, blending, drying, and conveying, without any of the bulk product being exposed to personnel or the environment.

By simplifying the process, powder and bulk solid producers can increase the quality of their products from raw material receipt and formulation to end product manufacturing and distribution--all without increasing manpower requirements—and, in many cases, allowing manpower to be reallocated to more productive tasks.

The Many Benefits of Precise Moisture Control
Although the reasons for measuring the moisture content of powders and bulk solids can vary, the primary motivation is to improve product quality and the bottom line.

As all bulk processors know, selling water is much more profitable than selling solids. Monitoring moisture content in all stages of production ensures the most efficient processing. From the measurement of incoming materials to in-process measurement, the optimization of plant resources and product quality will be ensured.

A prime example is extending the shelf life of powders in particular food ingredients. When moisture content is too high, mold can develop; when it’s too low, the product can become stale and the taste altered from ideal.

Establishing the moisture content is also very important when blending or mixing two substances together. If the mixing is not done at the proper moisture levels, the way the two products react to each other can be affected. This includes any chemical reactions that might take place, how the product flows to the next stage (in particular, packaging), and the way the product is perceived when received by the end-user or the next recipient on your supply chain.

It is also important to know the precise moisture content in any raw materials prior to beginning the manufacturing process. Otherwise, the product’s time in the dryer, the dryer temperature, the conveyor belt speed, and many other factors must be modified each time a new shipment of the product is introduced.

Another benefit of frequent moisture measurement is for products sold based on regulated moisture content. Prescribed percentages must be met to comply with these specifications. In certain industries, heavy fines could be levied, while in others, the product or substance will not be accepted by the regulating agency. These industries include food and pharmaceutical manufacturers, among others.

There may even be legal ramifications if the acceptable moisture content of a product is decided prior to purchase or shipping. Fees can be levied on companies that do not ship at the agreed moisture level or the product could be rejected outright upon receipt.

Finally, since moisture content contributes significantly to the weight of such materials, properly drying a substance to acceptable limits before it is transported can dramatically reduce shipping and disposal costs.

Simplifying Moisture Measurement
Although traditional laboratory and online-based moisture measurement techniques are useful in the right settings, they have lacked the simplicity and flexibility required for frequent spot checks.

One common test is loss on drying, which measures the total material weight change after drying. However, such tests typically require a sample to be prepared and brought back to the lab. The test takes at least 15 minutes to several hours to perform, which is too long when more immediate measurements are required. It also requires the sample to be altered or destroyed.

The other standard test is a Karl Fischer Titration. This method uses chemical “reagents” to break down the material and wick the moisture into a separate cell where the amount of moisture is then compared with the initial mass or volume. This test also takes time, some amount of technical expertise (to configure and maintain) and alters/destroys the test sample. It also only uses a very small sample making it questionable unless the manufacturing process produces extremely consistent product.

As a result, secondary test methods have typically been used to deliver faster results. This type of test uses an indirect method and a single conversion to achieve accurate results. Secondary measurement techniques are routinely accepted as equal to the gold standard method.  Examples are speedometers, common infrared and liquid thermometers and most pressure gauges. If there is a disadvantage, it is that the instrument must first be calibrated to ensure accurate results. In some cases, calibration could only be performed by trained staff familiar with the equipment.

In response, industry innovators have developed a simplified approach that allows even less-trained personnel to take portable, instant moisture readings of powdered bulk product as needed.

The approach involves moisture meters that utilize near-infrared (NIR) light, a highly accurate, non-contact secondary measurement method that can deliver immediate, laboratory quality moisture readings.

“NIR moisture meters allow very accurate instant measurement of solids and liquids without contact or sample preparation, so there is no contamination in handheld and online models,” said John Bogart, managing director of Kett US, a manufacturer of a full range of moisture and organic composition analyzers. “Once the meter has been calibrated against the lab or production standard, the calibration is stored in the device so no calibration is required in the field. Measurements are fully traceable to the original measurement method.”

In addition, because the process is non-destructive, samples remain unaltered, so they can be used for additional tests or put back into the product stream.

“NIR moisture meters follow the principle that water absorbs certain wavelengths of light,” said Bogart. “The meter reflects light off the sample, measures how much light has been absorbed, and the result is automatically converted into a moisture content reading.”

Traditionally, this technology has been utilized for desktop systems and online systems (over a conveyor line). The desktop system still requires an operator to remove a sample from the process and place it on the desktop unit. Unlike the LOD and KF methods, the measurement is quick (instant) and non-destructive.  

Online systems provide continuous measurement and can be utilized to optimize the process (speeds/feeds/temperature). However, this design requires the instrument to be able to “see” the product--perfect when the measurement is on a conveyor.

However, with today’s focus on tighter environmental controls, many bulk processors are moving to “closed loop” processing, where the product is processed from receiving to packaging without being exposed to the open-air environment. This manufacturing philosophy also helps optimize yields as none of the product is wasted. One drawback is that the traditional “online” meters cannot directly “see” the product. Autosamplers, sight windows, and other means can be used but, these create additional maintenance and costs. These additional costs and potential failure points reduce one of the main benefits of moving to continuous measurement, eliminating manpower needed to conduct repetitive sampling and testing.

Using an inline system, where a probe is inserted into the process, allows the bulk processor the ability to continuously measure the product in-situ so the various phases of the process can be optimized, without any manpower. Unlike complex laboratory equipment, in-line NIR equipment is designed for ease of use. Utilizing a fiber-optic probe either in contact with the moving product or watching the solids flow past, an inline instrument “sees” the process, measures the moisture and organic composition. Results, accurate to 0.001% in a 0-100% measurement range are shown on an integrated display and simultaneously reported back to the PLC for remote process monitoring and automated control. Robust system design allows for CIP (clean in process) so the probe doesn’t need to be removed.

Benefits of the “inline” systems are that measurements can be taken at a variety of locations, not able to be monitored previously. These include mixers, blenders (blend homogeneity), spray dryers, fluid-bed dryers, pneumatic/vacuum conveying systems, and more.

Because of integrated referencing, no direct contact or sample alteration is required, and particle size variation and unusual textures are not an issue. This can be important when used with a range of powder and bulk solid materials in different settings. For ease of use, the unit is operated via user friendly menu commands.

“The goal is for any staff member to be able to successfully use the moisture meter wherever it is needed, with minimal required training,” said Bogart. “This allows powder and bulk solid processors to have the certainty that what they are producing is of the highest quality.

“The key is to cost-effectively be able to conduct as much testing as required, with full confidence in the results, each and every time,” added Bogart.

Jim Roberts is territory manager, Kett US (Villa Park, CA). For more information, call 800-438-5388 or visit www.kett.com.

More Articles That May Interest You:

Harsh Environment Moisture Analyzer

On-Line Moisture Analyzer

Moisture Analyzers Feature Enhanced Process Security