PROCESS ANALYSIS | Sensors

S.B. Adeloju , in Encyclopedia of Analytical Science (Second Edition), 2005

Online moisture sensors

Moisture determination is an important aspect of many industrial processes. The range of available methods for moisture measurement in granular materials and the principles of these methods have been reported in a recent review. Of these, near-infrared (NIR) spectroscopy has gained most use for online moisture measurement in a wide range of granular materials. Also commonly used for moisture measurement in diverse range of granular materials are microwave phase shift (MPS) and microwave attenuation (MA) methods, radio frequency transmission, neutron moderation activation, and direct physical measurement (based on drag force principle) methods. Some of the specific applications of the methods are summarized in Table 3.

Table 3. Some industrial applications of online moisture sensors

Industrial applications Online moisture sensors/methods Best method
SGM MPS MA NIR CC RFT NMA LR DPM
Carpet √ √ MPS, MA
Cement √ √ √ DPM
Ceramics √ √ √ √ MPS, MA
Coal and minerals √ √ √ √ √ √ MPS, MA
Food products √ √ √ √ √ MPS, MA
Gases √ NIR
Grains √ √ √ √ MPS, MA
Paper fibers √ NIR
Pharmaceuticals √ √ MPS
Pigments √ NIR
Textile √ RFT
Tobacco √ NIR
Wood products √ √ √ √ √ MPS, MA

Moisture sensors/methods: standard gravimetric method (SGM); microwave phase shift (MPS); microwave attenuation (MA); near-infrared (NIR); capacitance and conductivity (CC); radiofrequency transmission (RFT); neutron moderation activation (NMA); low resolution (LR); direct physical measurement (DPM) based on the drag force principle.

Evidently, MPS and MA are the best methods for online measurement of moisture in most materials. However, for materials where most methods are not applicable or have not been attempted, NIR spectroscopy is the method of choice.

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FOOD AND NUTRITIONAL ANALYSIS | Overview

V. Jain , K. Gupta , in Encyclopedia of Analytical Science (Second Edition), 2005

Moisture

Water content/moisture content is the most ubiquitous substance in nature, the largest single constituents of all living things and affects quality, value, and freshness of food and is of major concern in food, paper, and plastic industries. Moisture determination is a widely used fundamental analytical operation, which satisfies the technological, analytical, commercial, and regulatory necessities in the processing, testing, and storage of food products and is an index of economic value, stability, and nutritional quality of food products.

Removal of water for processing/storage purposes either by conventional dehydration or freezing and drying alters the native functional properties of foods. Simple, rapid, and accurate methods for moisture determination in raw, processed, and stored food products are used to know the nutritive value of food products. A homogenous food sample should be prepared using a number of electrical/mechanical devices like blenders, graters, grinders, homogenizers, and mincers for the determination of moisture by any of the analytical methods given in Table 6, which are classified as direct and indirect procedures. The weight of sample is taken before and after it is dried, and the moisture content is calculated. Instruments used for moisture determination are simple to use and provide rapid and reliable measurements and are suitable for routine quality control applications.

Table 6. Classification of analytical methods for moisture determination

Direct methods Indirect methods
Direct methods usually yield accurate and absolute value and are manual and time consuming Indirect methods are rapid, nondestructive and easily automated
Physical and electrical methods
Drying method
Chemical desiccation AC and DC conductivity
Freeze-drying Dielectric capacitance
Oven drying Microwave absorption
Vacuum drying
Spectroscopic methods
Distillation method IR absorption
Azotropic distillation Near-IR reflectance
Chemical methods Nuclear magnetic resonance
Generation of acetylene Neutron and γ-ray scattering
Heat on mixing with H2SO4 Refractometry
Karl Fischer titration Sonic and ultrasonic
Extraction method
Gravimetric method
Thermogravimetric analysis
Absolute methods
Dew point method
Gas chromatography
Manometric method
Psychrometry
Volumetric

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COAL AND COKE

J.A. Pajares , M.A. DiezDíez , in Encyclopedia of Analytical Science (Second Edition), 2005

Sampling

Coal sampling may be defined as the extraction of a small amount of material from a larger bulk of coal such that the sample extracted is representative, as far as possible, for all analytes of interest. Sample preparation, which may involve drying, crushing, and subsampling, is an integral part of the sampling process.

The compositional variability of the material is such that coal sampling becomes a complex and difficult operation. The various standardization organizations around the world have all issued detailed documents that specify the conditions and methods necessary to obtain representative coal samples for analysis.

The process of sampling, which may vary from a 2   ton domestic consignment to a 165000   ton export shipment, involves obtaining a number of increments (spot samples, the product of a single action of the sampling device) that are combined to form one or several gross samples. Gross samples may then be crushed and subsampled (subdivided) to provide the analytical sample.

Coal samples may be obtained by either manual or mechanical sampling devices, the latter being more appropriate where large tonnages are experienced (e.g., at coal shipment terminals with throughputs up to 10000   tons   h−1) and where continuous operations or operator safety dictate their use.

Standard methods for obtaining coal samples specify minimum numbers of increments required to form a gross sample, based on a consignment (unit) mass of 1000   tons. For consignments of greater mass, the number of increments may be increased to provide a larger gross sample, or the consignment may be considered as several subconsignments, each of 1000   tons. For each of them, a separate gross sample is collected, to be individually processed and analyzed.

As well as the mass of the consignment, the required minimum number of increments is influenced by

•

the quality of coal – washed or cleaned coals are more homogeneous and therefore require fewer increments than blended or untreated coals and

•

the sampling process to be used – stopped-belt or falling-stream sampling requires fewer increments to attain the required precision than sampling from barges or stockpiles.

Sample preparation involves a series of operations such as reduction of size, homogenization, and reduction of the mass of the gross sample to that suitable for analysis. For the determination of size distribution, no sample preparation other than air drying and reducing the mass of the gross sample is undertaken. For other analysis, the extent of sample preparation is dictated by the intended analysis. For example, when testing for Hardgrove Grindability Index (ISO 5074, ASTM D409 or equivalent), a subsample of ∼1   kg coal prepared to a top-size of 4.75   mm is required, whereas for general analysis a final sample of 50–100   g crushed to less than 0.2   mm is sufficient.

Sample preparation procedures typically involve a fixed sequence of operations:

1.

Extract a subsample for total moisture determination.

2.

Air dry the gross sample (to ensure that the coal will flow smoothly through subsequent equipment). It should be noted that forced drying can have adverse effects, especially on coking properties; thus heating samples to more than 15°C above ambient temperatures should be avoided.

3.

Reduce the particle size of the sample. The whole gross sample is crushed to some intermediate size (10, 5, and 3   mm) using a mechanical mill or crusher.

4.

Mix the whole sample thoroughly to ensure homogeneity.

5.

Reduce the mass of the gross sample (sample division) to a mass consistent with the present size of the coal, by using a mechanical sample divider.

6.

Further crush the sample using a high-speed impact mill to attain the required particle size.

7.

Finally, mix and divide to the mass required for the laboratory sample.

It should be noted that the coking properties of crushed coal samples deteriorate rapidly with time. Therefore, samples intended for such testing should be prepared immediately before analysis.

Coke sampling is marginally less problematic because the product from a single source derives from coal or blend of coals that have been prepared to a specification for ash, moisture, particle size distribution, etc. The final coke produced will be relatively homogeneous in all properties, with the exception of size distribution. Standard methods are available for coke sampling that reflects the somewhat less rigorous requirements for this material.

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Water determination

Nilusha L.T. Padivitage , ... Daniel W. Armstrong , in Specification of Drug Substances and Products, 2014

11.5.1 Water determination by NIR

The determination of water, or moisture, by NIRS is attractive because it is one of the few analytes that displays strong absorption bands, which are often well resolved. 81 There are five bands of importance, namely 760, 970, 1190, 1450 and 1940 nm. The position of these bands may vary depending on the chemical and physical matrix of the sample. 78 Most publications, however, make use of ranges that include, at least, the 1450 nm (overtone) and 1940 nm (combination) bands. 18,19,77,82–90 In the pharmaceutical industry, moisture determination by NIRS has found application in the following areas/products: granulation, 83 lyophilization, 19,81,85,88,89 capsules (both hard and soft) 82,87 , differentiation between surface and bound water 84,90 and other drying processes. 83 NIRS is particularly powerful in that it is readily amenable to at-line, in-line and on-line analysis.

NIRS, however, cannot be considered a primary method and in order to function quantitatively it requires a reference method. 79 Examples of (primary) reference methods are those that have already been discussed in this chapter, namely KFT, GC and LOD. The values generated by the reference method for a given set of samples are then used to create a calibration model for the NIR instrument. The goal of this calibration model is to predict the moisture content of unknown samples from their NIR spectra. 69,70 Firstly, a model is generated by using a reference sample set, called the calibration set, which is analyzed with the reference method and mathematically correlated to its NIR spectra. Typically, an NIR spectrum needs to go through a data pretreatment and regression step as well (Section 11.5.2). Secondly, the calibration model needs to be evaluated with a validation sample set. The validation sample set assesses the NIR calibration model's ability to predict the moisture content in unknown samples. It is important to ensure that the calibration and validation sample sets are independent of each other and span the necessary concentration range for the desired application. Once the calibration model has been validated it can operate on a routine basis. Ideally the calibration model is used with the instrument that was used to construct it since transferability of models between instruments often requires further adjustments.

Analysts need to ensure that the calibration set is updated periodically and that the primary reference method is operating within its statistical parameters. The validity of the model can be regularly assessed by using standards.

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Nuclear magnetic resonance (NMR) imaging for evaluation of reinforced concrete structures

B. Wolter , in Non-Destructive Evaluation of Reinforced Concrete Structures: Non-Destructive Testing Methods, 2010

18.4 Application possibilities

18.4.1 Determination of moisture depth profiles

Usually, OSA–NMR equipment, such as NMR-INSPECT, does not achieve the sensitivity and spatial resolution of conventional MRI equipment. Whereas MRI provides 3D-resolved images with a resolvable linear increment in the sub-100   μm range, OSA–NMR is used to determine 1D-profiles with an increment in the 1   mm range. However, OSA–NMR offers the possibility of on-site and online inspection of moisture profiles. With OSA–NMR, the current moisture situation and variations in moisture distribution during wetting and drying can be observed directly on the building component. Entire building constructions can be inspected in a completely non-destructive manner, without the need to impair their integrity or their appearance by taking a sample.

The diagram in Fig. 18.4 shows a typical calibration curve, representing the functional dependence between the amplitude A of the 1H-NMR signal measured with NMR-INSPECT and the water content in lightweight concrete, expressed in mass-percent. The correlation is almost proportional. Such a calibration curve can be applied for almost all mineral construction materials. Depending on the measuring time, the inaccuracy of determined water content is typically between 0.3 and 1 mass-%. Moisture values as low as 0.5% can still be detected.

18.4. Moisture content measurement in lightweight concrete with OSA–NMR; calibration to gravimetric (destructively) determined moisture content.

In Fig. 18.5, the application and results of a moisture profile measurement on a lightweight concrete pillar are presented. The dotted line in the diagram shows the moisture profile as it was determined on-site. The profile could be determined up to x max = 26   mm. The entire profile measurement, with a resolved depth-increment of 1 mm and an inaccuracy in moisture determination of about 0.5%, took approximately 30  min.

18.5. (a) Moisture profile measurement on a lightweight concrete pillar; (b) the on-site measuring result (dotted line) as well as the moisture profiles (black line) and the integral moisture values (dashed lines) from laboratory measurements.

In order to verify the measurement result, a drilling core of 150   mm in length was extracted and investigated in the laboratory later. The sample was cut into three pieces of about 50   mm in length. By measuring from the top side as well as from the bottom side of each piece, it was possible to determine the moisture content at each point of the drilling core, despite the limited maximum measuring depth of 26   mm.

This 'laboratory' moisture profile is shown as a black line in the diagram of Fig. 18.5. Apart from small differences at the beginning, probably as a result of the one-day delay between on-site measurement and sampling, the good correspondence between the 'on-site' profile and the 'laboratory' profile is obvious.

It should be noted that the strongly decreased moisture contents at about 50 and 100   mm are a consequence of the cutting process. Finally, the integral moisture contents of the cut pieces were determined by gravimetry (weighing, drying and reweighing). The moisture contents determined by this method are shown as grey horizontal lines in the diagram.

18.4.2 Determination of liquid transport coefficients

Existing methods for non-destructive characterization of the durability properties of porous building materials, such as concrete, are regarded as being unsatisfactory. A key attribute for the evaluation of concrete's durability is its resistivity to the ingress of chloride and sulfate ions. It has been shown that these substances invade the interior of the concrete structure by piggyback transport with water. 21 Therefore, the main important long-term damage mechanisms in these materials are affected by the distribution and transport of water through the pore system. Hence, the determination of liquid storage and transport parameters is of fundamental interest for predicting the durability and service lifetime of these materials.

Nowadays, laboratory NMR equipment is routinely used to investigate the absorption and redistribution of water in different building materials. In this way, it is possible to determine the liquid transport coefficients and storage parameters for an exhaustive variety of materials. 22 These coefficients can be used as a basic data set for computer programs, allowing precise one- and two-dimensional calculations of simultaneous heat and moisture transport in building components, even under complex conditions. 23 Alternatively, OSA–NMR offers the possibility of determining these coefficients for concrete directly from the component.

Samples of concrete with various water transport properties were produced for an experimental study. The different numbers of capillary pores in the cement stone resulted from different water-to-cement ratios of the mixture, w/c = 0.45, 0.50 and 0.55. Additionally, the number of open pores was varied by changing the post-curing conditions, i.e. the samples were stored for 24   h at different temperatures (20, 45, 80 and 105   °C). Finally, the samples were exposed to water, which was applied without (capillary sorption) and with pressure (permeation). This moisture loading was interrupted at several points in order to monitor the moisture profile and the progress of the moisture diffusion front, respectively, with OSA–NMR.

Monitoring the time-dependent water uptake during capillary sorption allows the evaluation of the concrete's water uptake coefficient U, which is a measure for the absorption velocity. Firstly, U could be determined by the usual destructive method, i.e. by gravimetric determination (weighing, drying, reweighing) of the absorbed water amount in the sample at different times. Secondly, it was possible to determine the value of U by OSA–NMR. In Fig. 18.6a, both methods for water uptake coefficient determination are compared. An excellent correlation can be observed.

18.6. Non-destructive determination of liquid transport coefficients with OSA–NMR: (a) the water uptake coefficient U, (b) the water permeability coefficient K L.

For the determination of the water permeability coefficient, the concrete is exposed on one side to water, which is applied with a specific pressure p and for a specific time t (Fig. 18.6b). The depth of penetrated water is determined as a function of p and t. It is necessary to do this using several different water pressures and times in order to follow the procedure of the standardized method. 24 In principle, such an investigation can only be done non-destructively. It should be noted that one sample of concrete had an embedded steel bar, representing the reinforcement in concrete. The influence of ferromagnetic reinforcement is described in 18.5.

18.4.3 Early-age concrete hardening

Even though a variety of methods to measure the properties of fresh concrete are already available, monitoring strength development in early-age concrete is still an unsolved testing problem. During cement hydration, part of the mixing water is chemically combined and the residual water is confined in pores, which gradually decrease in size. These processes strongly affect the NMR relaxation times T 1, T 2. The same microstructural processes are also responsible for the development of the mechanical properties in cement stone. Therefore, measures of changes in T 1 and T 2 should be correlated to the development of strength and tightness in fresh concrete. The hardening of an individual concrete component can be monitored continuously by using OSA–NMR.

The hardening behavior of five different concrete mixtures as well as pure cement pastes was investigated. Different water-to-cement ratios (w/c) as well as the enrichment of some samples with a retarder should provide a wide range of hardening behaviors. Starting with their preparation, the development of the T 2 relaxation curve of each specimen was observed over three days.

Figure 18.7a shows the evolution of the T 2 relaxation curve for a single sample. It is very evident that the curve's decay is accelerating as hardening, i.e. cement hydration, proceeds. Fitting the experimental data to a one-exponential approximation function provides the time constant of the decay curves, which is the relaxation time T 2. This relaxation time typically decreases greatly between the beginning and the end of hardening, (Fig. 18.7b). For concrete without a retarder (S1, S3 and S4), this decrease is moderate in the first 2–3   h after preparation. This induction period is followed by an accelerated decrease (acceleration period) up to 20–30   h and finally the T 2 decrease slows down again (decay period). Hence the T 2 evolution follows the qualitative behavior of hydration progress and strength development. As expected, the acceleration period is greatly delayed for specimens with a retarder (S2 and S5).

18.7. Monitoring of concrete hardening by T 2 relaxation curve measurements: (a) as hardening proceeds the rate of decay of the T 2 relaxation increases, which is equivalent to (b) the decrease of the relaxation time T 2 as a function of hardening time.

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A review of high shear wet granulation for better process understanding, control and product development

Binbin Liu , ... Ruofei Du , in Powder Technology, 2021

4.7 Other monitoring techniques

Other techniques reported to monitor the HSWG process include vibration analysis, microwave measurement, and drag flow force (DFF) measurement. The correlation between vibration characteristics and particle states in the granulation process was explored very early [173]. Ohike et al. [174] converted the vibration wave into element versus frequency using the Fourier transform (FFT) technique, and proved the applicability of the technique to a concurrent endpoint monitoring system of granulation. In addition, Briens et al. [155] measured the vibration of particles in the granulation process at the 10-L and 25-L scale by utilizing an accelerometer placed on the outside of the bowl wall of the granulator. The results show that only the 25-L scale could be effectively monitored, while at the 10-L scale, the average frequency of vibration demonstrated the sensitivity to the process, and different experiments showed high variability. The difference between these two scales is believed to be caused by the mass of the material: The larger mass will have a greater impact on the vibration of the motor and the granulator bowl [122].

Microwave sensor systems have been shown to be a rapid and reliable tool for moisture determination in solid materials, including pharmaceutical granules. Gradinarsky et al. [ 123] confirmed the application of microwave measurements in determining the initial moisture content of a formulation and monitoring the initial phase of the HSWG process. This method basically presents the same advantages as the spectrometric method, such as non-destructive, non-contact, and high accuracy, but it has some restrictions on the determination of moisture content of materials. Research showed that the measurement range was limited to maximum of 14% relative moisture [123], while others complained about the non-linear behavior of measured parameters already at a 7% moisture content [175]. Therefore, expanding the range of moisture content measured by microwaves is a difficult problem that needs to be solved, especially for the HSWG systems with high moisture contents. Peters et al. [176] developed a new type of multi-harmonic sensor system, which shows advantages over existing systems; however, its application in practice needs to be further verified.

A DFF sensor is an analytical tool to measure the force exerted by wet massing in a granulator on a thin cylindrical probe. It can provide high frequency and high resolution in-line granule densification data that correlates well with product properties, such as granule densification and tablet dissolution [13]. Currently, DFF sensors have been used to fingerprint HSWG processes to aid process monitoring, scaleup, and granulation endpoint control [13]. In another study, Narang et al. [124] used a DFF sensor to measure the change of wet mass consistency in the granulation process, and verified it using an on-line FT4 powder rheometer. The result showed that there was a good correlation between the two measurements, and indicated that the DFF sensor can be used to monitor the change in material properties (e.g., shear viscosity and granule size/density) in the granulation process.

The appropriate selection of monitoring technology is conducive to improving the consistency of batches and better meets the requirements of product quality. All of these technologies have shown some promising applications; however, each technology has its own unique considerations, which hinder their widespread adoption. Therefore, the improvement of existing technology or the development of more reliable and direct technology has always attracted great interest among pharmaceutical scientists.

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