Process Gas Chromatographs. Tony Waters

Figure 1.2 Basic Gas Chromatograph.

Figure 1.2 introduces the essential hardware devices found in any gas chromatograph:

       A column oven with one or more controlled temperature zones.

       A carrier gas supply and pressure control system.

       A sample injector to inject a repeatable volume of sample into the flowing carrier gas.

       One or more separating columns.

       One or more detectors.

      All gas chromatographs have these basic functions, yet we see a large variation in their design and fabrication.

      The process instrument

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.

The oven

      Temperature control

      The hardware devices used by a gas chromatograph and the separations that occur within its columns are sensitive to temperature change, so a gas chromatograph needs very fine temperature control.

      In the first makeshift gas chromatographs the temperature‐controlled enclosure was literally a laboratory oven, and the name stuck; the column compartment of a gas chromatograph is still the column oven.

      Early PGCs had a single isothermal oven that housed the sample injection valve, column, and detector; and sometimes the pressure regulator too. The temperature setting was then a compromise that didn't always satisfy the needs of the individual devices. More recent instruments include several temperature‐controlled zones for columns, valves, and detectors, thereby allowing individual temperature settings.

      The chromatographic columns are very sensitive to temperature change. A change of column temperature will change the time that a component spends in that column, which might cause an error in analyte detection and measurement. Most columns today reside in a separate column oven often controlled to better than ±0.03 °C.

      Temperature programming

      A separate column oven may also support temperature programming, a sometimes‐useful technique that gradually increases the temperature of a column during analysis. When temperature programming is employed, the analyzer needs a reproducible cooling system to rapidly lower the column temperature to its original starting point.

      Temperature programming is common in laboratory gas chromatographs and allows them to separate a wide range of components, but it's rare in process gas chromatograph due to cost and analysis time issues. This may change with the introduction of less complex methods of heating and cooling, as discussed in Chapter 11.

The sample injection valve

      Laboratory and online practice

To produce a chromatographic separation, the instrument needs a small sample of the gas or volatile liquid for analysis. The introduction of this sample into the carrier gas stream is known as sample injection. After injection, the carrier gas carries the sample into the column. As used here, a volatile liquid is one that will rapidly and completely vaporize at the injector temperature.

      It used to be a standard laboratory practice to inject samples manually, using a glass syringe, but this routine procedure is now automatic. In the laboratory, an autosampler accepts an array of small vials containing the liquids for analysis. Then, according to a preloaded time program, it pulls a sample from each vial in turn and injects it into the chromatograph.

      In contrast, an online gas chromatograph needs to periodically extract a minute sample from a continuously flowing process fluid and inject that sample into the carrier gas flow. To do this, most PGCs use a mechanical sample injector valve having a pneumatic actuator powered by an air signal from the chromatograph control unit. A few use electric power.

Figure 1.3 shows a typical valve configuration for injecting gas samples. For clarity, the diagram shows a rotary valve, but there are several other types of valve in use, including slide valves, diaphragm valves, and plunger valves. Chapter 8 details the function, design, and usage of these valves.