On-line measurement techniques

Automation of process control has created a need for continuous monitoring of the particle size of particulate matter in process streams. Some on-line particle size analysis instrumentation has been developed recently to meet this need it can initiate regulatory or shut-down signals in control systems. The basic requirements for such instrumentation are that it must operate automatically and continuously under preset instructions, and the response time from observation to readout must be so short as to be nearly instantaneous. 

This relatively new range of techniques follows the general pattern found in the whole subject of particle size measurement a variety of techniques differing in 

2 suitability for different systems, materials and particle sizes  

4 the number of points on the distribution they are capable of determining  

Some methods only give a measure of central tendency (a mean diameter), others give one or more points on the size distribution. They may be truly on-line in operating on the whole process stream, or they may need a partial sample stream taken off the main stream or they may merely be automated rapid-response batch techniques. 

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The same accuracy as with the laser-scanner can be reached with thermocouples. However this is only a quasi-on-line measuring technique. The number of measuring points is limited by the number of thermocouples. The greatest disadvantage of this method is its interference during the build up of the crystal coat. 

Objective analysis of the composition of the phases developing on-line measurement technique fermentor-interface, instrument (computer) Difficulty great number + variety of components/selective analytical method not sensitive to interferences separation before detecting sterile conditions (built-in sensors, airtight scaling, endurance of strerilization) on subcellular level biochemical informations are limited... 

For any process, the lack of accurate process measurements limits the successful implementation of model identification and control. Crystallizers are dispersed-phase systems, and the shortcomings of on-line measurement techniques for these types of systems are particularly evident. 

Development of new methods and instrumentation became one of the core activities of the SM T programme. These developments concern innovative instruments necessary to improve the quality of measurements in particular sectors (e.g. on-line measurement techniques, field-measurement methods etc.) [10]... 

The need to understand metal extraction with ScFs, particularly uranium SFE, motivated the development of an on-line measurement technique for uranyl complexes in ScF CO2. In SFE processes, the extracted material is usually collected by depressurizing the ScF, followed by chemical or spectroscopic analysis. On-line monitoring of SFE can be preferable because it is fast, free of collection loss, and allows monitoring of process dynamics. However, depending on the specific analyte and the measurement matrix, the desired levels of sensitivity and selectivity can be difficult to achieve with on-line methods, particularly with detectors that can be coupled with pressurized ScF systems. 

On-line measurements are the key to implementing any kind of control strategies. Despite numerous efforts towards development of in situ biosensors, their use in bioreactors is still limited. Currently, only a few parameters, such as temperature, pH and dissolved oxygen concentration are measured using in situ probes. However, significant advances in developing and applying the new on-line measurement techniques have been achieved recently. 

Arnold M, Kajolinna T (2010) Development of on-line measurement techniques for siloxanes and other trace compounds in biogas. Waste Manage 30 1011-1017... 

Process control and instrumentation aspects. Important problems include off- and on-line measuring techniques for fine particles and their structures. In parallel with the better established characterization methods for particle size, shape and... 

On-line GPC technique for measurement of peak molecular weight of polystyrene... 

Charge measurements, as mentioned above, were also performed using the porous Pt electrodes required by the on-line MS technique. At low methanol concentrations (10 2 M), the charge ratio QaJQm, near 1 indicates that (C,0, H) must be the predominant adsorbate composition [14,47], This result is in good agreement with that of Heitbaum and co-workers [15] who used Eq. 1.2 to determine the number of electrons, n, per C02 produced from methanol adsorbate. They found for n a value of 3, which would be in agreement with reactions 2.1 or 2.2 for methanol adsorption. 

With the advance of computer techniques, especially implementation of distributed control systems (DCS) to chemical processes, a large set of on-line measurements are available at every sampling period. The rational use of this large volume of data requires the application of suitable techniques to improve their accuracy. This goal has triggered the focus on research and development, during the last ten years, in the area of plant data reconciliation. Complete reviews on the subject can be found in the works of Mah (1990), Madron (1992), and Crowe (1996). 

The studies described above have the purpose of identifying the reacting species in a solvent extraction process and developing a quantitative model for then-interactions. The fundamental parameter measured is the distribution ratio, from which extraction curves are derived. Solvent extraction work can still be carried out with simple batchwise (or point-by-point) technique, but continuous on-line measurements give faster and more accurate results. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

It is useful to give a brief overview of some of the major advantages and limitations of ultrasound as a tool for characterizing the properties of food materials. Ultrasound is fairly inexpensive to purchase and operate, it is robust and can therefore be used in factories, it is capable of rapid and reliable measurements, in a non-destructive and non-invasive manner. In addition, measurements can easily be automated and so the technique is suitable for on-line measurements as well as an analytical instrument in the laboratory. The major disadvantages... 

In comparison with other techniques, the major advantages of ultrasound are that it is nondestructive, rapid, and can easily be adapted for on-line measurements. 

Turbidity spectra and size exclusion chromatography (SEC) are two techniques which could be used for the on- and off-line monitoring of polymer particle size during the polymerization. An experimental evaluation of the two techniques is reported herein and it is shown that they satisfactorily follow the change of the particle size during the reaction. Information from SEC analysis is exploited to experimentally evaluate the turbidity spectra technique and investigate its potential to be used as an on-line measurement of particle size for latex reactor control. 

The development of hyphenated techniques has depended on advances in at least three areas interfacing, scanning speed of the measuring technique, and adequate data systems. The different combinations of chromatography and on-line measuring instruments have different requirements, but a few generalizations can be made. Since the GC/MS combination is the most popular and successful, it will be described as a typical example of the types of considerations required for interfacing instruments. Problems associated with other interfaces will be mentioned briefly later. 

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