Column selector valves

Column selector valves can be added on as accessories to allow column switching for multi-dimensional chromatography or for automatic column selection to facilitate methods development. Column switching valves should be located inside the column oven if possible. 

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Generally, this mechanism could be extended to accommodate any number of vessels with ease. For example, if 60 vessels were desired, 5 column-selector valves and 5 solenoids would be needed. Size limitations with regard to pump volume (or type, a reciprocating pump would not have a volume limitation) and oven size would be have to be overcome. 

Multi-solvent pumps, photodiode array detectors (PDA), modeling software, automated development systems, and multi-column selector valves... 

Briefly the positions of the valve are used in a manner illustrated in Figure 3. Position 1 is fitted to a 1/16" stainless steel tee (T3) fitting for delivery of fluid from the system syringe pump to the two 6-port tandem selectors for 12 column operation. In the 6 column system, this tee is eliminated. If six column operation is desired on the twelve column system, an on/off valve placed in line with one of the exit tubes (OU-4) from the stainless steel tee is manually closed and single delivery to one tandem selector is achieved. Position 2 of the VALCO selector valve delivers modifier from the liquid pump to each one of the extraction vessels automatically. 

With our design, there are two ways by which the selectors could be utilized to conduct extractions from a number of columns. The first utilizes a concurrent 2N non-stop mechanism, where N is the number of extractions vessels per column switching valve, for actuation and column selection. The second utilizes a concurrent 2N-1 stop mechanism. N is equivalent to the number of vessels. Thus, in the 2N mechanism, and for two tandem column selectors, 12 vessels can be selected. In the 2N-1 mechanism, 11 vessels can be selected. 

With the 12-vessel extractor, the 1/8" valve receives the extraction effluent from the vessels in tandem column selectors 1 and 2 (TCS-1 and TCS-2) into two separate ports 1 and 4 as shown in Figure 7. During the static mode, the counter-current valves, i.e. modifier pump valves (MP-3 and MP-4) are closed. Pressure build-up for static extraction then follows. Valves MP-3 and MP-4 are mounted close to the ports so that no accumulation of extract occurs. The valves are connected via a stainless steel tee (T2), to the modifier pump which is also used for flushing the lines after the extractions have been conducted. In the dynamic mode, extract flows from the unblocked ports of 1 and 4 to ports 5 and 6 then through to the delivery nozzles. 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

Valve V3 is set to allow gas to exit the selected adsorber through the back pressure regular (BPR) as soon as the desired operating pressure is achieved. For depressurization, V3 is switched to allow the column exit gas to bypass the BPR and be vented through valve V4. V4 is a vacuum/vent selector and V5 allows a single pressure transducer to monitor system pressures at up to four selected points. 

America, San Jose, CA, USA, and Scientific Software Inc., Pleasanton, CA, USA, versions for Hitachi LaChrom and LaChromElite hardware and EZChromElite software were launched. Partnership with Agilent Technologies resulted in the creation of a powerful version that supports Agilent 1100 LC and LC/MS systems with six columns and twelve solvent selectors. Further systems supported by ChromSword Auto are Waters Alliance LC systems with Millenium and Empower softwares. ChromSword Auto also supports 2-8 column and 2-16 solvent switching valves for all HPLC systems described. 

Constraint control is another type of selector or override that is intended to keep the controlled variable near a constraining or limiting value. Chapter 19 discusses how constraints influence the selection of operating conditions and why it is necessary in many cases to operate near a constraint boundary. Riggs (1998) has described a constraint control application for distillation columns with dual composition control, where reboiler duty Qr controls bottoms composition of xb and reflux flow R controls overhead composition The reboiler becomes constrained at its upper limit when the steam flow control valve is completely open. Several abnormal situations can result (1) the column pressure increases, (2) heat transfer surfaces become fouled, or (3) the column feed rate increases. When the reboiler duty reaches the upper limit, it is no longer able to control bottoms composition, so constraint control forces one composition (the more valuable product) to be controlled with the reflux ratio while the other product composition is left uncontrolled (allowed to float ). Computer control logic must be added to determine when the column has returned to normal operation, and thus the constraint control should be made inactive. 

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