Solution concentration measurement real-time

In some manufacturing process analysis applications the analyte requires sample preparation (dilution, derivatization, etc.) to afford a suitable analytical method. Derivatization, emission enhancement, and other extrinsic fluorescent approaches described previously are examples of such methods. On-line methods, in particular those requiring chemical reaction, are often reserved for unique cases where other PAT techniques (e.g., UV-vis, NIR, etc.) are insufficient (e.g., very low concentrations) and real-time process control is imperative. That is, there are several complexities to address with these types of on-line solutions to realize a robust process analysis method such as post reaction cleanup, filtering of reaction byproducts, etc. Nevertheless, real-time sample preparation is achieved via an on-line sample conditioning system. These systems can also address harsh process stream conditions (flow, pressure, temperature, etc.) that are either not appropriate for the desired measurement accuracy or precision or the mechanical limitations of the inline insertion probe or flow cell. This section summarizes some of the common LIF monitoring applications across various sectors. 

The general operation of the pilot scale reactor has be previously described by Pareek et. al. [3]. However, modifications were required to allow the injection of the gas and liquid tracers, and their subsequent detection at the outlets. The liquid tracer, 5mL Methyl blue solution (lOgL" ), was injected via a syringe inserted into the liquid feed line. Outlet samples were measured with a Shimadzu 1601 UV-Vis Spectrophotometer at a wavelength of 635nm. A pulse (20mL) of helium gas tracer was introduced using an automated control system, with the outlet concentration monitored in real-time with a thermal conductivity detector. Runs were carried out based on a two-level... 

The most common methodology for measuring fast kinetics in real time is to perturb a system at equilibrium for a time duration that is much shorter than the relaxation kinetics that follow perturbation. This perturbation can be achieved by changing the concentration of chemicals through fast mixing (stopped-flow), changing the temperature of the solution (temperature jump), simultaneously changing the... 

Of particular importance are in situ SFM measurements, which allow real-time data collection during structure evolution [111, 112, 117, 133-135], Both concentrated block copolymer solutions [117, 136, 137] and block copolymer melts [111, 112] have been imaged in situ to access the microdomain dynamics. Figure 3... 

Concentration measurements of aqueous solutions of salt, sugar, antifreeze etc. Quality control of a number of industrial processes requires checking the concentration stability of different solutions and water soluble fluids. The off-line use of Abbe refractometers is sometimes an obstacle to real-time process control. Several researchers have tested different types of LPGs for the measurement of concentrations of aqueous solutions of sugar, salts, and antifreeze. Falciai et al. [16] used a UV written LPG (X = 1,530 nm, A - 400 pm) to measure concentrations of aqueous solutions of sodium chloride (NaCl) and calcium chloride (CaCl2) having refractive indices in the 1.32-1.42 range. In a later paper, Falciai et al. [17] also measured... 

More recently, flow titrations have been implemented in multi-commuted flow systems. With these systems, the exact amounts of the solutions involved can be modified in real time according to a concentration-oriented feedback mechanism. The flow titration really mimics a true titration because an analytical curve is not needed. A flow system exploiting a binary search to define the end point of an acid—base titration [324] is a good example of this approach. Stream directing solenoid valves were used to modify the sample and titrant volumes after every measurement via a feedback mechanism. Samples with concentrations within a range of two orders of magnitude could be titrated without modifying the flow manifold. 

We developed a submersible electrode assembly, connected to a 50 ft (24 m) long shielded cable (via environmentally-sealed rubber connectors), for the real-time in-situ monitoring of the TNT explosive in natural water [10]. Such sensor assembly consists of the carbon-fibre, silver, and platinum working, reference, and counter electrodes, respectively, and operates in the rapid square-wave voltammetric mode. Such a remote/submersible probe circumvents the need for solution pumping and offer greater simplification and miniaturization. The facile reduction of the nitro moiety allowed convenient and rapid square-wave voltammetric measurements of ppm levels of TNT. Lower (ppb) concentrations have been detected using a... 

As discussed in Section 8.3.2, in addition to the fast and frequent on-hne measurement, some measurements may be available at infrequent and/or irregular times and with significant delays. For example, there may be a combination of MWDs and PSDs measured off-line by chromatographic methods and monomer concentrations measured in real time by spectroscopic methods. In these cases, the so-called multi-rate state estimators maybe applied. In these estimators, the fast measurements are used to estimate the state variables that are observable, while estimation of the non-observable variables is obtained in open-loop mode. When the (infrequent) measurement becomes available, the close-loop estimator is used. Ellis et al. [102] and Mutha etal. [108] used a multi-rate EKF to estimate monomer conversion and average molecular weights in the solution polymerization ofMMA. 

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