Bypass-stream transport

Bypass-stream transport is a method for maintaining high sample transport velocity to minimize transportation lag. This method is used when samples are vaporized at the sampling tap and no facilities exist for returning the vapor to the process. If the sample bypass is piped to a drain or vent, this will not only waste the process material but might also pollute the environment. Therefore, the use of a fast bypass-return loop is preferred. After selecting the appropriate sample transport method, the sample time lag should be calculated and used in the tuning of the analyzer controller. 

A salient example from the process sampling realm is therefore given here. In the routine PAT laboratory it was desired to perform a mass reduction as part of the process of transporting a powder material. A bypass stream was established and on this the device depicted in Fignre 3.5 is snpposed to deliver one 10% snb-sample, another 25% snbsample, with the remaining 65% ( reject ), fed back to the main stream. 

Alternatively, for nonequilibrium process streams, where a pumped reactor sample recycle line is available, in-line fiber-optic transmission cells or probes (Figures 5.26 and 5.27) can be used to minimize sample transport. It is highly desirable that some form of pumped sample bypass loop is available for installation of the cell or probe, so that isolation and cleaning can take place periodically for background reference measurement. 

If the material to be removed is dust, the self-cleaning bypass filter with automatic blowback constitutes a potential solution, whereas in other instances, cyclone separators should be considered. In the former device (Figure 3.5), the process stream enters tangentially to provide a swirling action, and the cleaned sample is taken near the center. Transportation lag can be kept to less than 1 minute, and the unit is applicable to both gas and liquid samples. This type of centrifuge can also separate streams by gravity into their aqueous and organic constituents. 

The Liqui-Cel Extra Flow module of the CELGARD EEC (Charlotte, North California) is characterized by a higher mass-transport coefficient than parallel how conhgurations because of the presence of a central bufhe that forces the liquid stream (sent to the shell side) to how perpendicular to the hollow hbers. The central bufhe also minimizes the shell side bypassing the system has been designed for avoiding large pressure drops. 

IB. Inventory Control. The membrane-cell brine process loses water with the waste streams, by transport through the membranes into the cell catholyte, and through evaporation into the chlorine produced in the ceUs. Even with minimization of the waste streams, there is a constant need for makeup water. This can be added to the saturator feed or directly to the pump tank. The choice of the addition point will determine the size of the bypass flow requited to maintain the brine density. Addition to the pump tank is feasible only when its dilution of the brine is not excessive. One method for controlling the makeup water flow is to measure the total brine volume continuously and add water to maintain the desired amount This is easily done because there are usually only two or... 

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