Modern chemical and petrochemical processing plants are complex systems containing many steps (often called unit operations) involved in producing one or more products from various raw materials. In order to control the many processes, for both improved product quality and operational safety, many measurements are made at the different stages of processing. These measurements, either from simple sensors (such as temperature, pressure, flow, etc.) or from sophisticated chemical analyzers (providing composition of one or more components in the chemical stream), are typically used as inputs to process control algorithms to give a "snapshot" of the process operation and to control the process to ensure it is operating efficiently and safely.
Traditionally, most of the measurements (with the exception of temperature, pressure and flow) were performed "off-line" by taking a sample from the process and analyzing it in the laboratory. Beginning in the latter of part of the 1930s, a trend aimed at moving the analysis from the laboratory to the process plant began. With the advent of more sophisticated analyzers, this concept known as Process Analytics become much more prevalent in the 1980s and a new discipline called Process Analytical Chemistry (PAC) emerged which combined chemical engineering and analytical chemistry.
One of the main driving forces for PAC (See also: PAT) is to remove the bottleneck and time lag associated with sending the samples to the lab and waiting for the analysis results. By moving the analysis to the process, results can be obtained closer to real-time which effectively improves the ability for the control action to correct for process changes (i.e., feedback and feed forward control).
By far, the most common implementation of PAC (especially for more complex analyzers) utilizes what is known as extractive sampling. This typically involves the continuous (or sometimes periodic) removal of a small portion of sample from a much larger piping system or process vessel. This sample is then conditioned (filtered, pressure regulated, flow controlled, etc.) and introduced to the analyzer where the chemical composition or the intrinsic physical properties of process fluids (vapours and liquids) are measured. In industrial plants, the majority of sample systems and their related analyzers are installed in analyzer houses.
The hardware (traditionally metal tubing, compression fittings, valves, regulators, rotameters and filters) associated with extractive sampling is collectively referred to as the sampling system. Sample systems are used to condition or adjust the sample conditions (pressure, amount of particulate allowed, temperature and flow) to a level suitable for use with an analytical device (analyzer) such as a gas chromatograph, an oxygen analyzer or an infra red spectrometer. Despite the simple explanation just given, modern sampling systems can be quite large, complex, and expensive. The design features of analytical sample systems have changed little, when the discipline of Process Analytics began in Germany, right through until the present day. An example of an early analyzer and sample system used at the Buna Chemical Works (Schkopau, Germany), is shown in the following photograph. Process analytics remains exceptional in the fact that it is the last outpost of low level automation (retains manual adjustments and visible checks) within the process industries.