Multiport valves

Figure 4.1 Schematic diagram of a coupled column system. The first column (ID) is connected to the second column (2D) tlirough the interface or valve system. The interface can be a diiect coupling, a live T-union, a complex multiport valve, or a thermal or cryogenic modulation system. The stimulus can be the switching of the valve, abalancing pressure to divert flow towards 2D, an added flow that is used in pressure tuning, or the drive mechanism for the modulator. The line to detector 1 will normally be a non-retaining section of column. In a two-oven system, ID and 2D will be in different ovens the dotted line indicates separately heated zones.

It is preferable to sample vertical pipelines rather than horizontal lines (ideally when the flow of steam is downward rather than rising). Sampling in the vicinity of bends, elbows, and valves should be avoided. At the sample point, the multiport valve should be installed perpendicular to the flow of steam. 

Yoshida et al. developed similar numbered-up capillary pilot reactors for free radical polymerization (Iwasaki et al. 2006). The capillaries were either arranged in parallel fashion similar to conventional multitube reactors or consecutively branched by multiport valves. 

Where binary, ternary or quaternary gradient elution (p. 91) is required, a microprocessor controlled low-pressure gradient former is the most suitable (Figure 4.31(c)). The solvents from separate reservoirs are fed to a mixing chamber via a multiport valve, the operation of which is preprogrammed via the microprocessor, and the mixed solvent is then pumped to the column. For the best reproducibility of solvent gradients small volume pumps (< 100 gl) are essential. 

A dissolution testing apparams consists of a set of six or eight thermostatted, stirred vessels of an established geometry and volume from the USP guidelines. The dissolution apparatus provides a means to dissolve each sample, but does not provide a means to determine the concentration of the aetive ingredient in the bath. In the most well-established scheme, sipper tubes withdraw samples from each dissolution vessel and send them through a multiport valve to a flow cell sitting in the sample chamber of a UV-vis spectrophotometer. In recent years, moves have been made to make in situ measurements in the dissolution baths by means of fiber-optic probes. There are three possible probe implementations in situ, down shaft, and removable in situ (see Table 4.2). 

Most of the frequently used comprehensive 2D LC systems employ a microbore HPLC column in the first dimension, operated at low flow rate, both under isocratic and gradient conditions. This enables the transfer of fractions of small volume via the multiport valve equipped with two identical... 

The key part of the system is the multiport valve, which interconnects the different parts and solutions used by the system. The common port is connected to a reversible pump with the retention coil placed in between. The pump is connected to the carrier solution reservoir. The common port can access any of the other ports, which lead to sample, standard solutions or reagents, mixing chamber and sensor array, by electrical rotation of the valve. Since the system is bidirectional, volumes can not only be propelled directly to the detector, but also be injected into the retention coil, therefore merging accurate aliquots of different solutions. In order to assure proper mixing of the solutions not only via diffusion... 

In order to characterize the adsorbed species in the zeolite pores during the reaction, IR spectra of the catalyst on stream were recorded (time resolution 30 sec. for the first 5 minutes, 5 min. afterwards) after introducing the carrier gas stream containing the reactants (pressure transient response). The concentrations of the adsorbed species were derived from the integrated areas of the ring vibrations of the aromatic molecules (for details see ref. [13]). Simultaneously, samples of the effluent gas stream were collected into the 16 loops of the multiport valve and subsequently analyzed by means of gas chromatography. 

Another technique, which is much more frequently employed, relies on commercially available multiport valves with a large number of sampling loops (an example is a valve carrying 16 sampling loops, with 16 positions and 2 x 16 + 2 = 34 ports, i.e., two for each loop plus a gas entrance and a gas exit port). The simplest arrangement of such a multiport valve downstream of the catalytic reactor is shown in Fig. 7 (for the sake of simplicity, only two sampling loops are drawn). The multiport valve is assisted by an eight-port valve with two positions. The needle valve is to com-... 

Figure 7. Instantaneous sampling and product storage with a multiport valve. The parts within the area circumscribed by the dotted line are heated to avoid condensation of vapors.

Sample Injection. In principle, the samples can be injected in two ways with a syringe through a septum, or through a multiport valve at the channel inlet. The septum material must be inert to the solvent, but usually cannot withstand more than 30 punctures before replacement is necessary. Furthermore, the septum variant of injection complicates the entire FFF operation and is rarely used anymore. Sample injection by multiport valves does not suffer from these complications. 

An alternative for applications where pressure differences cannot be used is to employ a combination of valves or a multiport valve to direct the sweep gas into the injection coil, similar to operating a chromatography injection port. 

Manual and automatic valves of the diaphragm, butterfly, or ball type are commonly used of a construction suited to the application. Automatic valves may be actuated pneumatically, hydraulically, or electrically and where large engineered plants are concerned separate valves are ascribed to discrete functions. Smaller packaged units often employ a single multiport valve mounted on the top or the side of the vessel. 

At the end of the sampling period, the valve is re-opened and the normal carrier gas flow resumed. In this way, the sample is introduced onto the column without using a gas syringe, thus avoiding fractionation due to pressure changes in the syringe. Since the system is closed, no headspace is lost during the transfer. The equipment requires no multiport valves, which reduces the number of components that come into contact with delicate samples. 

TPD of basic probe molecules is a method which is often used for the analysis of zeolitic acidity [24], In a parallelized implementation [25] of this technique, the samples to be studied were placed in a parallel channel reactor-body built analogous to the one described by Hoffmann et al. [26], By means of a multiport valve, the effluent from each channel could be switched to a mass spectrometer which was used for the analysis of the desorbed gas. In a typical experiment, after conditioning of the samples and adsorbing ammonia, a temperature ramp was started and in a sequential manner the effluent of each catalyst was fed into the mass spectrometer. Flushing and analysis times per channel were 8 s, so that all 10 samples could be analyzed in somewhat more than one minute. This proved to be sufficient to obtain well resolved TPD curves at a heating rale of 10 K/min. The recorded data corresponded well to curves measured with conventional setups. 

Different implementations of such reactors may differ in detail. For instance, in some cases the flow to the analytical instrument is additionally controlled by a mass flow controller, so that at least during the analysis all catalysts are evaluated at exactly the same space velocity. In other setups, which will be discussed below, a truly parallelized analysis is integrated into the reactor, which allows the elimination of the multiport valve. The general features of such reactors, however, can be summarized as those of a multitubular fixed bed. Critical issues in operating such parallel reactors have been discussed by Moulijn et al. [33], who also had published one of the first prototypes of a parallel reactor, the so-called six-flow reactor [34],... 

A last problem is the fact that in zeolite catalyzed reactions often high boiling products arc formed, either as the desired product, but more often via unwanted side reactions. Such products can be deposited in the multiport valves and lead to clogging of individual ports and build-up of pressure. Most commercial multiport valves capable of handling high pressures cannot be heated to more than 80°C which is not sufficient for many problems. We are operating home built valves which can be used up to 100 bar pressure and a temperature of 200°C. Such systems, however, are complex and require a high level of maintenance. 

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