I. Gas Injection Valve Application The gas injection valve is not only used in gas chromatography for petrochemical analysis, but also widely used in headspace samplers, online monitoring and other instruments. In general, the application of gas injection valves is mainly divided into online and offline.
1. On-line mode In many online monitoring instruments, the sample loop of the gas injection valve is connected in series to the reactor to be monitored, allowing the gas to be tested to continue, continuously passing through the sample loop, and at intervals, the valve is automatically Switch once to introduce the gas in the sample loop into the system. For example, it is applied to automatic and continuous monitoring of ambient air.
Gas chromatography can also be cascaded in larger reaction systems in this way, such as in natural gas extraction and pipeline transportation in the petrochemical industry for on-line testing.
2, offline mode And we see more, is the offline application of the valve.
The sample is taken and transferred to the laboratory using a gas injector, gas sampling bag, sample cylinder, connected to the sample loop, and the sample is injected.
Second, the injection repeatability in the gas injection valve offline mode has a biggest problem, that is, injection reproducibility.
We know that gases are easily compressed relative to liquids. At different pressures and temperatures, even the same volume of gas, there will be no small difference in the amount of material. For example, we use a syringe to pump gas into the sample loop. First, the gas injector cannot be temperature-controlled. Even if the needle is heated many times and injected with a hot needle, the temperature cannot be kept constant and constant. Second, how much force we use to push the needle bar and inject the sample into the sample loop, which is not precisely controlled, that is, the sample pressure in the sample loop is not fixed. Third, the outlet of the sample loop is the atmosphere, and the atmospheric pressure is not constant. These aspects have led to the way of offline gas injection, and the reproducibility of injection is not particularly good.
On the other hand, on-line gas injection, the temperature of the sample is controllable, the speed of the gas passing through the sample loop is controllable, and the pressure at the outlet of the sample loop is controllable. These aspects make the reproducibility of the online gas injection much higher than the offline mode.
At the same time, people are also starting from these aspects, through better temperature, flow rate and pressure control, improve the reproducibility of gas injection. For example, some headspace injectors improve injection reproducibility by better temperature control of the gas injector.
Still other designs are at the outlet of the sample loop, with a back pressure valve to control the pressure of the gas inside the sample loop. In addition to improving reproducibility, this design also increases the pressure inside the sample loop, reducing the pressure difference between the sample loop and the gas phase system, thereby reducing the baseline fluctuations caused by pressure fluctuations during valve switching, and better. The peak shape of the peak of the sample is improved.
Of course, for gas samples, in addition to directly sampling the gas and then transferring it to the laboratory, it can also be collected by adsorption, including thermal desorption, solid phase microextraction, and the like.
Co2 Fire Valve,Co2 Fire Extinguisher Valve,Valve Fire Extinguisher,Fire Fighting Valves
NINGBO TOMAN IMP. & EXP. CO., LTD , https://www.tdotmfiresolution.com