Читать книгу Process Gas Chromatographs - Tony Waters - Страница 18

The early development of the gas chromatograph was unusual. After the invention of the technique in 1952, the oil and chemical companies soon recognized its potential for process control, and those industrial companies did much of the original development work. The contribution of the instrument manufacturers came later.

Consequently, gas chromatographs intended for process monitoring and control evolved differently from those intended for laboratory use. Although both types of instrument use the same core technology, their sphere of application is quite different.

For example, a process gas chromatograph performing a two‐minute analysis receives 720 samples per day. The laboratory chromatograph might only receive three.

Thus, the design specifications for a gas chromatograph installed in an industrial processing plant are quite different than for a gas chromatograph sitting on a laboratory bench. The main reasons for these differences are:

 The process instrument operates in a potentially hot, cold, dusty, wet, windy, corrosive, or hazardous environment.

 The process instrument operates continuously twenty‐four hours per day, seven days per week.

 The process instrument must operate reliably with almost no human intervention – perhaps only one calibration check each month.

 The process instrument can focus on measuring just a few of the components in a sample – the ones needed for process control.

 The process instrument suffers from a fanatical quest to reduce analysis time, so its measurements are valid for process control.

For all the above reasons, a process chromatograph (PGC) may include devices not shown in Figure 1.2. Later chapters will further discuss those devices. To whet your appetite, expect to see:

 Devices external to the instrument to condition the incoming process sample to make it compatible with the chromatograph; i.e. a sample conditioning system.

 Multiple columns with special valves to switch analyte molecules from one column to another, thus maximizing the rate that separated components arrive at the detector. This is an additional complexity rarely found in laboratory instruments.

 Housekeeping columns that allow strongly‐retained components to quickly exit the column system. A laboratory instrument used only a few times each day has plenty of time to recover between sample injections.

 Robust column systems and stable devices, all designed to operate for a long time without adjustment. In contrast, the laboratory staff can frequently check and adjust their instruments, as necessary.

 Automatic validity checking and automatic calibration as necessary. Most laboratories analyze a quality control sample every day.

 Hardened electronic devices to capture and process the detector signal and to schedule timed events.

 An analyzer enclosure, shelter, or house to protect the analyzers and workers from the plant environment.

The following paragraphs introduce the basic function of the hardware devices. Later chapters detail their performance and technology.