Sampling Systems for Process Analyzers

The sampling system consists of all the equipment required to present a process analyzer with a clean representative sample of a process stream and to dispose of that sample. When the analyzer is part of an automatic control loop, the reliability of the sampling system is as important as the reliability of the analyzer or the control equipment. 

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K.G. Carr-Brion and J.R.P. Clarke, Sampling Systems for Process Analyzers, Butterworth-Heinemann, Oxford, 1996. 

D. C. Cornish, G. Jepson, and M. J. Smurthwaite, Sampling Systems for Process Analyzers, Butterworths, London, 1981. 

System installation in a permanent location may require a sample conditioning system featuring some degree of automation, such as automatic cleaning (the system illustrated above features such a system) and outlier sample collection and the need to interface to an existing control system process computer. The latter may require that the system operates with a standardized communications protocol, such as Modbus, for the chemical industry. Certain specialized industries use different protocols, such as the semiconductor industry, which uses SECS and SEC-11 protocols. A standardized approach designated the Universal Fieldbus is another method/protocol for process analyzers which is being supported by certain hardware manufacturers. 

Using the above criteria, a number of online and offline explosion proof sampling systems for Hg in natural gas applications have been designed, and these consist of a multi-stream process analyzer, which performs inlet and outlet measurements across Hg removal beds. 

The on-line system is the most commonly used system for compositional analyzers. In this approach, the sampling system is automated and directly interfaced to the analyzer. With the analyzer system external to the process, it is fairly easy to repair and/or calibrate the system without the need to shut the process down. As this is the most common real-time analyzer approach, it will be discussed in more detail later in the chapter. The at-line process is similar to the old laboratory analysis in that a manual sample must be taken out of the process but in this case the analyzer is rugged enough to be located in the process area to do an immediate analysis once the sample is obtained. This method is labor intensive to do but does solve the sampling... 

There is a need in both the laboratory and process reactors to minimize the time delay between sample extraction and analysis results if the data are going to be used effectively to determine an end point or be used for feedback control. For these reasons, there has long been a desire for process analyzers to be able to collect data directly from the process stream itself without the need for a sample-handling system. Conceptually, one could simply mount the analyzer to the process stream to accomplish this goal. In reality however, safety, maintenance, environmental, and other practical concerns make this undesirable. 

A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

The problem is made more difficult because these different dispersion processes are interactive and the extent to which one process affects the peak shape is modified by the presence of another. It follows if the processes that causes dispersion in mass overload are not random, but interactive, the normal procedures for mathematically analyzing peak dispersion can not be applied. These complex interacting effects can, however, be demonstrated experimentally, if not by rigorous theoretical treatment, and examples of mass overload were included in the work of Scott and Kucera [1]. The authors employed the same chromatographic system that they used to examine volume overload, but they employed two mobile phases of different polarity. In the first experiments, the mobile phase n-heptane was used and the sample volume was kept constant at 200 pi. The masses of naphthalene and anthracene were kept... 

The modified FMEA approach has been used by the API to develop RP14C. In this document ten different process components have been analyzed and a Safety Analysis Table (SAT) has been developed for each component. A sample SAT for a pressure vessel is shown in Table 14-4. The fact that Tables 14-3 and 14-4 are not identical is due to both the subjective natures of a Hazard Analysis and FMEA, and to the fact that RP14C is a consensus standard. However, although the rationale differs somewhat, the devices required are identical. (The gas make-up system in Table 14-4 is not really required by RP14C, as we shall see.)... 

As a consequence, good, safe, steam-sampling points are required, and automatic, real-time continuous analyzer systems for monitoring of steam and condensate quality are very useful. These requirements usually are not a problem in larger power and process HP boiler plants. Here, each facility tends to have a unique combination of operating conditions and waterside chemistry circumstances that necessitate the provision of a steady stream of reliable operational data, and this can be obtained realistically only from continuous, real-time analysis. 

In several chapters we discussed how the quality of the analytical result defines the amount of information which is obtained on a sampled system. Obvious quality criteria are accuracy and precision. An equally important criterion is the analysis time. This is particularly true when dynamic systems are analyzed. For instance a relationship exists between the measurability and the sampling rate, analysis time and precision (see Chapter 20). The monitoring of environmental and chemical processes are typical examples where the management of the analysis time is... 

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