Pressure syringes

At the time of the solvent methanol experiments a metering pump was used. In some experiments the pulsating action of the pump can be disturbing, so a high-pressure syringe-type pump can be used. Since mass flow controllers are available now, the combination of a gas-pressurized feed tank on an electronic scale for liquid level indication and a mass flow controller seems to be a good choice. Both the feed tank and separator can be heated or cooled. In the case of the solvent methanol experiments. 

Figure 6.3 Schematic diagram of an on-line SFE-GC instmment 1, carbon dioxide 2, high-pressure syringe pump 3, tliree-poit valve 4, extraaion cell 5, oven 6, gas cliromato-graph.

The stop-flow technique is the simplest and least expensive method of injection. The flow of the mobile phase is first stopped, either by turning an on—off valve in the line before the column (with constant-pressure systems), or by stopping the pump (with constant-flow systems). The column then returns rapidly to atmospheric pressure, and the sample can be injected directly on to the column with a normal low-pressure syringe. 

Reactant feeds are generated by vaporizing liquid flows from HPLC pumps with manometric pulse dampeners or high-pressure syringe pumps and mixing this vapor with gas components metered through mass flow controllers. This reaction feed is then divided between the 48 channels equally by using flow restrictors such as silica capillaries or micromachined channels (Fig. 3.12). The capillaries feed into the inlet stand-offs of the reactor modules. 

Reactions run under a nitrogen atmosphere were arranged with a mercury bubbler so that the system could be alternately evacuated and filled with nitrogen and left under positive pressure. Syringes and reaction flasks were dried at least 12 hours in an oven at 120 °C and cooled in a desiccator over anhydrous calcium sulfate prior to use. Reactions at room temperature (rt) were conducted under ambient laboratory conditions T = 20-27 °C, P = 720-770 mmHg. References to removal of volatile components or concentrated under reduced pressure refer to rotary evaporation of a sample at 25-65 °C under pressure (15-25 mmHg) at room temperature. 

A high pressure syringe pump operated under constant pressure mode Is often used for low flow rate operation In narrow-bore packed column HFLC. Flow rates as low as 0.05 Mi /nln can be obtained. The drawback of constant pressure operation Is the potential flow rate variation due to a flow resistance change In the column. 

Figure 19-20. Diagram of a constant pressure syringe pump.

Before charging the system with the benzyl chloride, the reactor was heated to 75 °C and pressurized to 138 bar with CO2. Using a manual high-pressure syringe pump, 1.80 mL of benzyl chloride was introduced to the reactor, yielding an initial concentration of 0.055 M. Throughout the reaction, stirring was maintained at 200 rpm. 

Septum injection, pneumatic high-pressure syringe injection and v.alve injection are the usual kinds of sampling devices. The stop-flow injection technique is also used. Automated sampling devices based on high-pressure syringe injection or valve injection are already available. 

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