Selecting valve

Figure 13.3 Schematic diagram of the parallel cryogenic trap MDGC-IR-MS system A, splitless injection port B, RC-5 non-polar first-stage separation column C, HP 5970B MSD D, HP 5965B IRD E, four-poit two-way valve (300 °C maximum temperature) F, external auxiliary earner gas G, six-poit selection valve (300 °C maximum temperature) H, stainless-steel cryogenic caps I, tliree-poit two- way valve (300 °C maximum temperature) ...

There are a number of vendors that sell solenoid actuated Teflon valves, which are easily interfaced to a computer. Care must be taken to chose a design where the internal volume at the valve outlet can be flushed easily between steps, however [110], Rotary selection valves have been used as well, but given the number of rotations needed for a 200 cycle deposit, various failure modes revealed themselves. 

A Cooling Flow No 1. Control valve fails closed 1. Loss of cooling, possible 1. Select valve to fail open DAC 1/93 ... 

Since the eluent selection valve must direct the data-rich effluent from the LC system to the MS and divert the effluent from the other LC system to waste, the timing of the valve control must follow Equations 4.8 through 4.10. 

Programming — The third controller is programmed to trigger both LC systems, the start of MS data acquisition, and the direction of the column selecting valve. 

The following should be considered when selecting valves ... 

ViUeneuve, M.S., Anderegg, R.J. Analytical supercritical fluid chromatography using fully automated column and modifier selection valves for the rapid development of chiral separations. J. Chromatogr. A 1998, 826, 217-225. 

Modifier Pump. The first feature in our adapted design is the introduction of a liquid pump via an instrument controlled VALCO (Model E04, Valeo Instruments, Houston, TX), four position selection valve. We have used an LKB Model 2150, dual piston pump for pumping modifier and entrainer fluids (LKB-Produkter AB, Bromma, Sweden). However, any suitable liquid pump could be substituted. Only pure fluids such as carbon dioxide have been introduced with the Suprex system syringe pump. With the addition of this second pump to deliver liquids, modifier is introduced directly into the extraction vessel. A wide range of alternative fluids and fluid mixtures can be rapidly selected with this dual pumping option. The criteria for selection of a modifier pump include the ability of the pump heads to withstand pressures in the range of 100 to 300 atm and interfacing capabilities, i.e. the ability to be turned on and off by the Suprex contact closure controls. 

Static/Dynamic Selection Valve. This valve is the key feature of our design in that it eliminates the use of a restrictor. Restrictors are the most common means of controlling the pressure or density of a supercritical process. With no restriction, flow is dead-ended (i.e. restricted) via a switching valve in our invention. Supercritical fluid extractions are then conducted in a static (no flow) mode. 

Once the extraction is complete, the static/dynamic selection valve is repositioned to the dynamic mode to allow flow. Subsequently, pressure and density are rapidly reduced to prevent significant losses of the supercritical fluid from the syringe pump tank and the extraction effluent, which is being transferred for collection. With a non-re-stricted transfer, the flow of supercritical fluid effluent is rapid. This desire for rapid depressurization led to the development of a delivery nozzle which would ensure collection of the extracted solutes without losses. Details of this delivery system can be found in the next section. 

Figure 6. Static/Dynamic Selection Valve Setup for Six Vessel Multi-vessel Extractor. Extraction Effluent Received from One Tandem Column Switching Valve.

Integrated process in SBS format time sharing of MS by multiplex of HPLCs using multiple sprayers or stream selection valves online SPE... 

Secondary screening is carried out in parallel fixed-bed reactors sized for catalyst loadings between 15 and 500 mg. The system is a 48-channel reactor consisting of six modules of eight reactor wells. Six channels are analyzed in parallel and stream selection valves provide fluidic multiplexing between reactor vials and analytical hardware. A typical system is shown in Fig. 3.11. 

Each of the eight reaction product streams leaving the reactor module are fed into a Valeo stream selection valve where one of the eight effluents is selected and sent to a GC for on-line analysis. The remaining channels, combined, are sent through a back pressure controller to a waste. The selected channel from each... 

A technique called shave/recycle, mentioned earlier, allows separtion of a pair of close resolving peaks. To use shave/recycle, it is necessary to plumb the HPLC system so that the output from the detector can be returned to the HPLC pumps through small diameter tubing and switching valves (Fig. 11.2). Twenty-thousandths tubing is used to connect the detector output to valve 2, the waste recycle valve 0.02-in tubing connects from valve 2 to valve 1, the solvent select valve and, finally, a third valve 3, the collect valve, can be placed in the waste line from valve 2. 

When a specific valve design is selected for the application, the rated capacity of that valve can be determined using the actual orifice area, the rated coefficient of discharge and the equations presented in this book. This rated relieving capacity is then used to verify that the selected valve has sufficient capacity to satisfy the application. 

There are, however, a number of valve designs where this is not so. When the SRV is selected, therefore, the actual area and rated coefficient of discharge for that valve must be used to verify the rated capacity of the selected valve and to verify that the valve has sufficient capacity to satisfy the application. 

J A (Gas) — A (Liquid) Add areas, select valve with at least this area... 

The main requirement for thermal relief valves in cryogenic conditions is to reduce their freezing risks at any cost (Figure 11.2). Therefore, we must select valves with low simmer, a rapid pop/snap opening and high seat tightness. We need to reduce unnecessary product loss, so again low simmer and preferably a short blowdown is required. 

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