In-line Moisture Measurement Ensures Product Quality

A silica sand company located in the southern U.S. was looking for a method to validate the proper moisture content of its sand prior to it being size classified and packaged for its consumers. It was particularly critical that the moisture measurement be performed in-line (also referred to as on-line) within the process, thereby achieving real-time results without the delay associated with sampling systems.

The company felt that an in-line moisture measurement sensor would help achieve its goals. The moisture monitoring of the sand would assist in two important ways:

1. Prevent sand that is too moist from getting to the particle size screen and blocking it up. A blocked condition can slow down or even stop valuable processing time and possibly cause equipment damage.
2. Ensure sand being packaged was at the proper moisture content to achieve the best quality of the finished products for their customers.

The process consists of taking the raw sand from a receiving hopper and conveying the material to the dryer. After the drying is complete, the sand is belt conveyed to a bucket elevator system that brings the sand to the particle size screen. The sand is then sorted by size and sent to bins that dispense the sand for packaging.

The company did some research and decided to use the HumiCore PRO in-line moisture measurement system. Before the sensor was installed, the company and sensor manufacturer collaborated to determine the optimal location to place the sensor. The moisture sensor was installed immediately following the dryer using a sled to glide across the surface of the sand. At this point, the sand is being conveyed to the size-sorting screen. If the sensor detects a higher level of moisture in the material than desired, an alarm is sent to the control room. Then a chute can divert the sand back to the dryer before it is re-sent through to the sorting screen. Using an optional DIN-Rail transmitter, the company was able to have the output signal of the moisture sensor integrate easily with their other control functions. The HumiCore system has been successfully working as anticipated and the company is very pleased with the results.

In-line moisture measurement sensors used for process monitoring of product moisture in solids and emulsions applications are usually based off of a few common technologies. These technologies include near infrared (NIR), high-frequency field and microwave. The HumiCore Pro moisture sensor, chosen by this company, uses a circuitry principle that is centered around an electrical high frequency field. This is based on technology that has been developed and proven to be successful over several years.
    
There are several benefits of high-frequency field technology including:

1. Ability to measure moisture within the material core. In many applications, it is critical to measure not just the surface moisture, but rather the inherent (average) moisture through the core of the material. Drying processes extract moisture from material by exposing the material’s external surfaces with heat. Naturally the surfaces closest to the heat source will see a greater moisture reduction. The high-frequency field technology is able to penetrate the material nearly 8 in. so the attained moisture measurement is actually measuring the material core, a truer representation of the actual moisture content. This is a critical difference to near infrared (NIR) technology which is only able to measure the surface moisture of the material.

2. Ability to measure moisture regardless of particle size, particle shape, particle surface characteristic, or color. The high-frequency field technology is not affected by these material characteristics, unlike near infrared (NIR), therefore providing a reliable measurement for a wide range of applications. Infrared (NIR) technology is typically limited to measuring the moisture content of thin films or layers of material, paper, and other thin materials.
    
In general, continuous on-line moisture sensors can be used for most powders and bulk solids processes that incorporate conveyor belts, screw conveyors, pipes, chutes, etc. Given the capability of the moisture sensor, a control system can be connected to quantify and display the moisture content of the material passing by the sensor face instantaneously.

Common reasons for using these types of sensors include automating the drying or moistening processes of materials, improving final product quality by providing optimal moisture content, saving of energy costs by optimizing drying and/or moistening processes, and receiving real-time data without having to make numerous laboratory samples.
    
Some of the main criteria that the company was looking at when it chose the HumiCore PRO system were:

•    Ability to easily incorporate the sensor into the process and receive continuous, real-time measurements instead of having to take multiple laboratory samples
•    Capability to measure the moisture to the product’s core instead of just the surface to increase the consistency of the measurements
•    Ability to receive fast and precise measurements with accuracies typically around 0.1% to 0.3%
•    Durability of the sensor and the sensing surface to withstand the heat and abrasive characteristic of the sand
•    Capability to have the sensor output communicate with the existing control system to receive alarms and control the processes

In summary, an in-line moisture measurement sensor can be an easy way to incorporate a system that helps automate and verify the quality of end products. It can usually be added without requiring major changes to existing processes.
    
Monitor Technologies LLC is a manufacturer and supplier of level, moisture, and flow measurement sensors for powders and bulk solids. For more information, call 1-630-365-9403, e-mail [email protected], or visit www.monitortech.com

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