The preferred arrangement for a vacuum or a pressure column with a large amount of inerts is shown in Figure 3.10. Here the inerts are pulled off or blown out through a vent line in which there is a throttle valve manipulated by the subcooled-condensate temperature controller. For a vacuum column, the low-pressure source is usually a steam jet. If the downstream pressure fluctuates too much, it may be necessary to use a cascade temperature-vent flow-control arrangement.
For a vacuum column with a small amount of inerts, the arrangement of Figure 3.10 may require an unpractically small vent valve. In this event the arrangement of Figure 3.11, with a controlled bleed from the atmosphere (or source of inert gas), is better.
A more complicated but more flexible arrangement, such as that of Figure 3.12,* is well suited to either vacuum or pressure columns when the amount of inerts fluctuates over a wide range. It has worked well, for example, on a column in a semicontinuous process that is shut down and started up every day or so, and that must handle severe transients during the startup period. The vent line is connected to a pressure-dividing network with two control valves connected so that as one opens, the other closes. A split-range adjustment of the two positioners (see Chapter 11, Section 10) permits both valves nearly to close when the controller output signal is at its midrange value. Since the sum of the two acoustic resistances is always high, even though each valve is sized to handle a maximum flow equal to five to ten times the average, normal flow of air or gas through the two valves in series is economically small. When an expensive inert gas such as N2 must be used, it is common to minimize or eliminate split-range overlap to reduce consumption even further. For large columns that must be started up and shut down frequently, an additonal large vent valve is sometimes installed in parallel, and split-ranged with the small one. This facilitates getting the column on line at startup.
For many columns the vent flow functions primarily as a purge and is small enough that moderate changes do not affect column operation. In such cases a manually set vent or bleed valve is often adequate and no direct control of condensate subcooling is necessary. For other cases, where column feed rate varies significantly, the vent or bleed valves may be tied to the pressure controller to work in parallel with the condenser cooling-water valve. An example is shown in Figure 3.13.
It should be acknowledged that many engineers today prefer to control condensate temperature by manipulation of condenser cooling water; pressure is then controlled by (1) throttling the vapor takeoff if there is a large amount of inerts, or (2) throttling an air or inert-gas bleed if there is only a small amount of inerts. The objections to, and difficulties with, condensate temperature control, via condenser cooling-water manipulation, were stated earlier. As far
* The symbolism "AO" means air-to-open; "AC" means air-to-close.
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