Offline Measurements

Off-line measurements differ significantly from on-line techniques. Offline measurements include safety testing in a laboratory, which has the major advantage of providing available time to redesign the process and/or equipment if a risk is identified. Of course, immediate process response is not possible. 

An offline measurement apparatus is usually not directly mounted on the reactor, but is fed with samples withdrawn from it manually or automatically. This is the typical case of chromatography, a widely used measurement device for gas and liquid composition. Both gas and liquid chromatographies are based on the separation of the sample by means of selective adsorption on a solid substrate posed in a fixed bed column, and on the detection of the change of a suitable property of the (gas or liquid) carrier, usually thermal conductivity. 

This brief overview of offline measurements can be concluded by considering the measurements of the heat released by chemical reactions, which can be obtained via calorimetric measurements [7, 18]. The most diffused industrial calorimeters are the so-called reaction calorimeters, basically consisting in jacketed vessels in which the reaction takes place and the heat released is measured by monitoring the temperature of the fluid in the jacket. A class of alternative instruments are the scanning calorimeters (differential or adiabatic), in which the analysis is performed by linearly increasing the sample temperature with respect to time, in order to test the reactivity of potentially unstable chemical systems in a proper temperature range by measuring the released heat. 

To this purpose, a brief overview of the measurable variables in a batch reactor has been included the difference between online measurements, suitable for control purposes, and offline measurements, which can be exploited to obtain experimental data to be used for the identification, has been stressed. 

There has also been an increase in the ability to measure the intracellular pH, by flow cytometry with pH-dependent fluorescence indicators (Oz-kan and Mutharasan, 2002) and nuclear magnetic resonance. There are several reports that relate the intracellular pH value to cell behavior, including metabolism, apoptosis, and specific growth rate (Cherlet and Marc, 1998). Such offline measurements help to interpret the physiological state of the cells, but still cannot be used as control systems. 

As well as the estimation of the cell concentration, other parameters related to the biomass can be obtained. However, at present these are limited to offline measurements. Among these are ... 

One alternative to the direct online measurement of polymer properties is to use a process model in conjunction with optimal state estimation techniques to predict the polymer properties. Indeed, several online state estimation techniques such as Kalman filters, nonlinear extended Kalman filters (EKF), and observers have been developed and applied to polymerization process systems. ° In implementing the online state estimator, several issues arise. For example, the standard filtering algorithm needs to be modified to accommodate time-delayed offline measurements (e.g., MWD, composition, conversion). The estimation update frequency needs to be optimally selected to compensate for the model inaccuracy. Table 5 shows the extended Kalman filter algorithm with delayed offline measurements. Fig. 2 illustrates the use of online state estimator... 

Table 5 Extended Kalman filter algorithm with online and offline measurements...

With the rather unsophisticated instruments under discussion, background may be corrected for by scanning or by successive online and offline measurements. In general, scanning is needed when background is intense whereas the other method suffices for weak background. 

FTIR has also been used for online compositional analysis of polymer blends and copolymers from a plant extruder. Varying concentrations of styrene or acrylic polymer were identified from the resulting FTIR spectra. The online monitoring was accurate when temperature fluctuations were +5°C. The offline measurements were found to be less accurate than the online measurements. 

Materials such as wood, rubber, solid plastics, and other materials can also be nonsampled and analyzed directly. NIR analysis of this type is simplified by the fact that usually only one or two constituents are to be determined. Also there is a very strong signal at specific wavelengths, relative to the background, for the constiments to be determined. Conventional sampling of these materials must be used for reference analyses. Fiber-optics can now extend the range of offline measurements to several meters, and it is likely that both fiber-optics and interactance will find many applications in NIR analysis in industry in the future. 

In terms of response time, we have a compromise situation fast response times usually come from devices such as vibrating elements (e.g. the Nametre) placed directly in the flow line, however these might be difficult to relate to off-line measurement, and are certainly very sensitive to the local flow rate. On the other hand, the best devices that virtually duplicate offline measurements are usually much slower in response because of the relatively long time taken to replace the liquid being measured (e.g. the Brookfield TTIOO). Thus the actual choice of device must be made very carefully. 

What is clearly needed is a metric that could monitor the blend uniformity, in real time as the blend occurs. Ideally, such a process would be noninvasive to the blend process, as accurate as the offline measurement, and nonlabor intensive. There are a number of techniques that have been developed to serve as a real-time blend uniformity monitoring. Near infrared (NIR) spectroscopy is probably the most mature technology that has been applied to blend imiformity analysis and will be discussed in detail below. 

---