Determination of water in powder (and other) materials. Presented by: Jan Nielsen, Danish Technological Institute

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1 Determination of water in powder (and other) materials Presented by: Jan Nielsen, Danish Technological Institute

2 A short overview Definitions and state-of-the-art measuring systems Challenges The European project: Metefnet (Metrology for Moisture in Materials) The set-up at Danish Technological Institute and some results Next step in-line determination of water activity Dias 2 of 17

3 Metrology Here moisture determination is treated from a metrological point of view Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement Metrology may be divided into three basic activities: Definition of internationally accepted units of measurement. Realisation of these units of measurement in practice Application of chains of traceability linking measurements made in practice to reference standards (calibration). DTI operates metrology laboratories within the fields Temperature, Humidity, Flow, Pressure, Mass, Air velocity, Geometry and Electrical measurements Motivation? The most fundamental problem in moisture metrology is that it is method dependent and based on standardisation of procedures We work towards outcome-based verification of measurement results through meaningful calibrations with traceability to the SI Dias 3 of 17

4 Definitions Water in materials Water can be present in many ways e.g.: Absorption as a chemical reaction Binding hydrate absorption and formation Molecular diffusion in the structure Surface energy binding Simple surface water Moisture content is how much water there is in a given material. Water activity (aw) is how efficiently water is bound in a material and therefore how much free water there is available for chemical reactions and growth of microorganisms such as bacteria and fungi. Dias 4 of 17

5 Definitions Water in materials - continued Moisture content is at present defined volumetrically or gravimetrically: Volumetric: θ = V water V material +V air +V water Mainly relates to the Karl Fischer-method. Gravimetric (wet basis): θ = m water m wet = m wet m dry m wet Gravimetric (dry basis): θ = m water m dry = m wet m dry m dry Water activity (slightly related): a W = p p 0 The definitions does not differ if the water is free or bound but the measurement methods does Dias 5 of 17

6 Measurement methods some examples Direct Method Gravimetric (e.g.): Mass loss on drying method (LoD) Thermogravimetric analysis (TGA) Dynamic vapor sorption (DVS) Chemical (e.g.): Coulometric Karl Fischer titration (ckf) Challenges 1. It is not necessarily only water that is extracted from a sample and the sample is not necessarily completely dry during weighing after drying 2. Calibration of instruments is often unsatisfactory due to absence of appropriate reference materials and lack of calibration services (traceability to mass is not robust due to 1) 3. Sampling and handling of samples may introduce errors 4. Often the sample holder is small (homogeneity of sample) 5. Uncertainty of measurement is hard to assess 1. The degree of binding of water affects the results if the water is released from the sample by heating. 2. If the solid sample is dissolved in a suitable dry solvent, interfering compounds in the sample may significantly affect the results. 3. Calibration of instruments is often unsatisfactory due to absence of appropriate reference materials and lack of calibration services 4. Sampling and handling of samples may introduce errors. 5. Uncertainty of measurement is hard to assess

7 Measurement methods some examples Indirect Method Spectroscopic e.g. Near infrared (NIR) Microwave absorbtion Challenges 1. Measures only surface moisture moisture gradient causes problems 2. Reflektive surfaces or significant colour variations in material can be a problem 3. Calibration of instruments is often unsatisfactory due to absence of appropriate reference materials and lack of calibration services - not robust due to 1) 1. Very dependent on material and temperature 2. Calibration of instruments is often unsatisfactory due to absence of appropriate reference materials and lack of calibration services - not robust due to 1) Dias 7 of 17

8 Challenges The use of different methods and definitions yields different results Sampling and handling of samples is an important error source The quality of moisture measurements in solids is therefore assured by reference moisture determinations according to standardised procedures. There are more than 1300 documentary standards (ASTM, ISO and CEN) in active use containing procedures related to moisture content determinations. In-line measurements can be performed using microwave or NIR techniques, but the final quality control of the product is most often made by mass loss on drying or titration methods. SI traceability through calibrations, would improve the quality of moisture control and enable a leap forward in developing real time process control based on on-line moisture measurements. Therefore the project: Metrology for moisture in materials Metefnet funded by European Metrology Research Programme (EMRP), implemented by European Association of National Metrology Institutes (EURAMET) - a consortium of 8 Metrology Institutes (incl. DTI), 3 Universities and 16 collaborators (companies, organisations and non-european institutes). Dias 8 of 17

9 EMRP Call 2012 SI Broader Scope SRT-s23 Metrology for Moisture in Materials ( Collaborators: Since there is no SI traceability, comparing results is difficult initiatives of METefnet would be extremely welcome [GEA] Aim of the project: to enable dissemination of SI traceability to moisture measurements in industry throughout Europe by removing ambiguities and inconsistencies in moisture measurement and calibration techniques. Develop new more relevant and effective methods of realising and disseminating SI units of moisture and provision of metrology infrastructure for moisture measurements. What are DTI targeting in the project. Determination of water content in a (reference)material, SI traceable to dewpoint temperature via a chilled mirror hygrometer. Calibration of or other dissemination of traceability for customers transfer standards To be able to simulate a moisture value according to a given standard and determine the error compared to the true moisture value Dias 9 of 17

10 The DTI Set-up A homemade primary standard comprising an LoD system (oven drying and weighing) and a chilled mirror based water vapour detection system. A novel combined sample containment system / measurement chamber has been constructed for contamination free in-situ sample handling, with a capacity for samples up to a mass of 200 g and volume of 200 cm3. The target moisture content range for the primary standard is from 10 g/kg to 950 g/kg. The target relative uncertainty for LoD measurements is 0.5 % or better and 2% for water vapour detection. Temperature range: room temperature to 180 C Dias 10 of 17

11 The DTI Set-up - continued The sample is placed in the measurement chamber in a temperature-controlled chamber that may be heated up to 180 C. Dias 12 of 17

12 The DTI Set-up - continued g/m 3 The sample is placed in the measurement chamber in a temperature-controlled chamber that may be heated up to 180 C (or 105 C for a biomass sample). Dry air is directed into the measurement chamber through a heat exchanger, drying the sample, and to a chilled mirror hygrometer type MBW 373, monitoring the dewpoint of the output air over time. The dewpoint is converted to absolute humidity (g/m 3 ). Dias 13 of 17

13 The DTI Set-up - continued F P T g/m 3 F, T, P m 3 The sample is placed in the measurement chamber in a temperature-controlled chamber that may be heated up to 180 C (or 105 C for a biomass sample). Dry air is directed into the measurement chamber through a heat exchanger, drying the sample, and to a chilled mirror hygrometer type MBW 373, monitoring the dewpoint of the output air over time. The dewpoint is converted to absolute humidity (g/m 3 ). The volume of the gas flowing over the sample is determined from the measured flow rate, the air temperature and the pressure. Dias 14 of 17

14 The DTI Set-up - continued F P T g/m 3 F, T, P m 3 g The sample is placed in the measurement chamber in a temperature-controlled chamber that may be heated up to 180 C (or 105 C for a biomass sample). Dry air is directed into the measurement chamber through a heat exchanger, drying the sample, and to a chilled mirror hygrometer type MBW 373, monitoring the dewpoint of the output air over time. The dewpoint is converted to absolute humidity (g/m 3 ). The volume of the gas flowing over the sample is determined from the measured flow rate, the air temperature and the pressure. The water loss can now be determined by integrating the obtained drying curve over time. Dias 15 of 17

15 The DTI Set-up - continued The sample is placed in the measurement chamber in a temperature-controlled chamber that may be heated up to 180 C (or 105 C for a biomass sample). Dry air is directed into the measurement chamber through a heat exchanger, drying the sample, and to a chilled mirror hygrometer type MBW 373, monitoring the dewpoint of the output air over time. The dewpoint is converted to absolute humidity (g/m 3 ). The volume of the gas flowing over the sample is determined from the measured flow rate, the air temperature and the pressure. The water loss can now be determined by integrating the obtained drying curve over time. The detachable measurement chamber with the sample is weighed before and after the drying, in order to determine the weight loss or optionally it is removed from the temperature-controlled chamber during the drying period in order to monitor the weight loss over time and enable this curve to be compared with the curve obtained for the water loss. Dias 16 of 17

16 Measurement on milk powder moisture content Drying temperature 87 C (as ISO 5537) Typical moisture content approx. 4 % Comparison with traditional LoD measurement < 1% difference Dias 12 of 17

17 And something completely different wood pellets Dias 13 of 17

18 A few remarks on water activity The water activity (aw) is a measure of how efficiently water is bound in a material The water activity is related to the equilibrium relative humidity over the sample, when placed in a closed container aw=0 bone-dry, aw=1 pure water aw can be determined indirectly by measuring the relative humidity (ERH) above a sample in a temperature stabilised sealed chamber. Water activity for many products is more or less temperature dependent. It is not straightforward to quantify the effect or even to predict if an increase in temperature will increase or decrease the water activity. The relationship between the water activity and the moisture content can be characterised at a given temperature (moisture sorbtion isotherm) And based on such characterisation the temperature effect on a specific product may be modelled or determined experimentally and corrected back to room-temperature conditions if needed Dias 14 of 17

19 Measurement on milk powder water activity Dias 15 of 17

20 Water activity measurement on-site version Prediction model + software results in measurement periods of a few minutes Uncertainty of the measurement system is approx aw in the temperature range 10 C to 40 C. The chamber can handle large samples and the system can be used in an industrial environment.. An automatisk version is at present under development by an industrial partner. Dias 16 of 17

21 Next step Metrology for Humidity at High Temperatures and Transient Conditions, granted under EMPIR Call 2014 Industry SRT-i EU contribution 1,5 M DTI aim to develop a practical method for temperature correction of in-line water activity measurement via modelling based on moisture sorption isotherms. We will introduce new services concerning traceable determination of moisture sorption isotherms up to 70 C. By combining traceable continuous temperature and sample based sorption isotherm measurements with the models we are going to develop a new method for traceable measurement of water activity in-line. Technical University in Darmstadt develops an absolute dtdlas-hygrometer (dtdlas = direct tunable diode laser absorption spectroscopy) with response time less than 1s. With this we can measure the humidity, spacial and temporal variations over a sample. The dtdlas will be used for water activity measurement via the above techniques. This completes the package: SI traceable measurement of moisture content and water activity reference values SI traceable measurement of moisture sorption isotherms Basis for in-line measurement of water activity at high and variation temperatures. Dias 17 of 17