The need for a soft sensor...
As refineries and their products become more widely used, the stringent specifications for their quality and refinery products also become tighter.
Crude oil, the starting point of all refinery streams, comprises scores of pure compounds. As such, quality specifications for various refinery streams are built with end-users in mind, i.e. RVP is specific for gasoline; taking into consideration safety in transport, vapor lock in gasoline feed system, type of storage tank employed, starting characteristics of motor fuels, etc. It is rarely an indication of the exact composition of the specimen; this becomes the responsibility of the ground staff (operating personnel) to meet the quality specifications.
Typically, staff have knowledge of the temperature, pressure, flow, DP, etc., as indicators of quality and general health of the plant. The actual point of interest, the quality itself, is only obtainable by lab analysis. Laboratory tests are designed so that they are fast, easy to execute, and repetitive. Depending on the nature of test, results can be derived in anything from a few minutes to a few hours.
As quality requirements become increasingly important, analyzers and GCs also shift from the lab to the field. Currently, there are several companies offering a wide range of easyto-
maintain instruments capable of measuring a variety of properties and numerous other features. There is no doubting the accuracy of the measurements delivered by these, but inherent in the very same instruments is the maintenance that is required.
Elaborate arrangements have to be made to clean and flush them (particularly for those that measure properties in more than one stream), and to provide housing. Safety is also a
key consideration for these housings and equipment within as all these factors affect the all important time delay between sensing and delivering instruments.
All of these were necessary till process engineers and mathematicians synergized their knowledge to introduce a fresh lease of life into process quality measurement.
