Specific sensors are the simplest type of online analyzers. They are generally used to measure either the physical parameters of a gas or liquid stream, such as pH, temperature, turbidity, or oxidation-reduction potential, or easily detected compounds, such as oxygen, cyanide, or chlorine. In most cases, these sensors have continuous output.
Sensors are relatively inexpensive and easy to install and require little or no maintenance. While they can be selective and sensitive, specific sensors can become fouled by constant contact with process materials, particularly those with high particule concentration.
While the output from specific sensors is considered to be in real time, all process analyzers, including sensors, experience lag times and stabilization times. The lag time is the time required for the process material to pass through the sensor's sampling element. The stabilization time, called T90, is the time required for the sensor to reach 90% of its final output. Typical T90 times are about 20 to 60 sec.
GAS CHROMATOGRAPHY (GC)
GC is one of the most widely used analyzers in the petrochemical and refining industry. It offers flexibility of applications, high sensitivity analysis, and multicomponent analysis. The widespread use of GC is a result of its versatility. If a sample can be vaporized, an effective separation is often possible.
Process GC sampling is extractive. A small sample of process material is obtained with a sample valve and vaporized in a preheater. The sample is pushed by an inert carrier through a packed solid capillary tube. Components within the sample have varying degrees of affinity for the column packing. The more the component is attracted to the column packing, the slower it moves. As the components come out of the column (that is, are eluted), they are recorded as a function of time by a detector. Depending upon the sensitivity of the detector (see Figure 3.12.3) and separation quality of the instrument, GCs routinely achieve accuracy within 0.25 to 2%. The accuracy of most analyzers is a fixed percentage of their full-scale range.
GC offers continuous results, but the retention time of the sample (i.e., the time taken for the separated components to pass down the column) must be considered. Typical retention times range between 1 and 20 min, depending on the number of components and their vaporization temperatures. A retention longer than 20 min is impractical and generally unacceptable for online analysis.
A big drawback of GC is maintenance. Instruments generally have many electrical and mechanical components. Preventive maintenance is a must. Repairs are not always fast or easy. Repairing a chromatograph can take hours or even days.
LIQUID CHROMATOGRAPHY (HPLC)
HPLC is similar to gas chromatography. HPLCs also use carriers (mobile phase), columns, and detectors. The sample is passed through the column under high pressure. However, because the HPLC column is much smaller in diameter than the process GC column, plugging results if the liquid sample is not free of particulates. Not many process streams can be conditioned to provide the necessary clean sample. To date, HPLCs have not been used with great success in liquid component process analysis.
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