PULSED PISTON FLOWS

Welschof (1962) carried out tests on dense phase plugs having a low velocity of 1 m/s. Later Lippert (1965) did an systematic analysis of the plug flow behavior. Following these researchers in the plug field were Weber (1973), Konrad et al. (1980) and Legel and Schwedes (1984). In 

Konrad was the first to address the issue of pulsed piston transport using the properties of the solids as they slide through the pipe in a plug-like motion. The friction generated in such systems often can be likened to bin and hopper flow and design, requiring shear stress measurements such as carried out by the Jenike shear stress unit. The final expression using the Konrad approach can be written for horizontal flow as 

The stress terms F at the front of the plug and B at the back of the plug, dependent on the pressure drop and powder properties, can be developed from a momentum balance but often times they are set equal to each other. Using the momentum balance 

The term c is zero for cohesionless powders such as plastic pellets. As one sees, there are several parameters that need to be measured specifically 

Weber (1973) has put forth a rather simple approach to pulsed piston modeling which couples the gas and solid behavior. The expression that Weber suggested is given in Eq. (9). One notes that this expression has an exponential decay of the pressure as the plug length decreases. This behavior has often been seen in industrial operations. 

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The technology that was proposed in the mid 1970 s essentially consisted of the production of a pulse-piston flow technique with alternating plugs of solids and air... 

Rotating-disk and pulsed columns Continuous, piston flow Dispersed, piston flow... 

Provides a constant and almost pulse free flow. Both pump chambers are driven by the same motor through a common eccentric cam this common drive allows one piston to pump while the other is refilling. As a result, the two flow-profiles overlap each other significantly reducing the pulsation downstream of the pump this is visualized below. 

Changes in the water table of the Mohawk River and a number of adjacent observation wells is reported in Fig. 4.10, adapted from Winslow et al. (1965). The wells followed the river, with a time lag of 4-12 hours (insert in Fig. 4.10). Two possible explanations for this time lag may be envisaged (1) arrival of the hydraulic pulse, or (2) arrival of the recharge front (assuming piston flow section 2.1). To tell the two apart, the time lag observed for these wells by temperature measurements is helpful, as discussed in section 4.8 (see Fig. 4.21). The temperature time lag of, for example, well 58, has been observed to be about 3 months, whereas the water table time lag was only 12 hours. The latter defines the arrival of the hydraulic pulse, whereas the former defines the travel time of the recharge front. The distances given in the insert in Fig. 4.10, divided by the respective time lags, provided the... 

A pump delivers the mobile phase through the chromatographic system. In general, either single-piston or dual-piston pumps are employed. A pulse-free flow of the eluent is necessary for the sensitive UV/Vis and amperometric detectors. Therefore, pulse dampeners are used with single-piston pumps and electronic circuitry with dual-piston pumps. 

Even in a large fluid bed reactor, the average residence time of the gas is quite short, i.e. a few seconds or tens of seconds. The distribution of gas residence times has been studied by Danckwerts et who concluded that it corresponds much more closely to piston flow than to complete mixing. It is possible to pulse the air supply in order to compress the range of gas residence times and to increase the average residence time. This technique also allows a wider range of particle sizes to be accepted than would otherwise be possible in a fluid-bed process. 

Even with such careful design of the cam drive mechanism, single reciprocating piston pumps produce significantly pulsed eluant flow-rates and hence some dampening or other appropriate course of action must be taken by the instrument maker to overcome this limitation since otherwise they would not be suitable for use in HPLC. 

To minimize pulses in the reagent flow, for the reasons already mentioned, a syringe pump is normally used for the reagent supply. Alternatively, if a piston pump is used, then an efficient pulse dampener should be inserted between the pump and the mixing T. 

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