Low rate operation

Vessels for the separation of two immiscible liquids usually are made horizontal and operate full, although some low rate operations are handled conveniently in vertical vessels with an overflow weir for the lighter phase. The latter mode also is used for particularly large flows at near atmospheric pressures, as in the mixer-settler equipment of Figure 3.19. With the usual L/D ratio of three or more, the travel distance of droplets to the separated phase is appreciably shorter in horizontal vessels. 

Attention to low-rate operation is the key to avoiding freezing problems. The following techniques can be used. 

Minimum acceptable throughput tests are often conducted when the column is anticipated to operate at low throughput for lengthy periods. The tests supply information on whether column internals should be modified or replaced to improve performance (e.g., reduce energy consumption) during low-rate operation. 

Consideration must be given to the thermal state of the feed this state should either be kept constant or be adjusted in the manner anticipated under low-rate operation conditions. Often, spare capacity is available in a preheater or a precooler when the column operates at low rates, and this can be utilized. 

If the heating medium is fouling, the above control systems may lead to reboiler plugging or fouling during low-rate operation, when reboiler velocities are low. A system similar in principle to Fig. 17.7a can help boost velocities. 

This system can be made to work in several cases if designed carefully. The key point to consider is low-rate operation, and to ensure that under such conditions both velocity and cooling water outlet temperature are satisfactory. The following guidelines for using this technique have been recommended (68) ... 

During low-rate operation, an inert gas can be injected into the process side of the condenser (173). This lowers the heat transfer coefficient. The controller will compensate by raising water flow into the condenser. The greater water flow will lessen the outlet temperature. Successful application of this technique has been reported (173). 

The advantages of the system are fast response and the ability to run the condenser and reflux drum at a lower pressure than the column. The latter may be a distinct advantage when the colunrn is revamped to operate at higher pressure. Because of its fast response, at least one designer (258) feels that this is the best method when tight pressure control is required with total condensers. Others (e.g., 77, 164) do not favor this scheme because of its high costs and its potential problems during low-rate operation. 

Only specific incidents were included. For instance, a statement such as "Leakage from chimney trays in refinery, vacuum columns can be reduced by seal-welding tray sections does not constitute a case history. On the other hand, a statement such as "One vacuum column experienced severe chimney tray leakage at low-rate operation. Seal-welding tray sections reduced leakage to acceptable levels does. 

Consider low-rate operation when designing spargers. 

Refinety Crude tower Column pressure and product gas rate sharply fluctuated during low-rate operation in this and other luiits. The condenser was a partial condenser located at ground level, with an elevated reflux drum. Problem was caused by slug flow in the riser from the condenser to the drum. Size risers to avoid sltig flow even during low-rate operations. 

Pay attention to low-rate operation of control systems throttling cooling water to condensers. 

A straightforward way of accomplishing this (at least from a battery design perspective) is to put together two cells with different rate capabilities, such as Li/ I2 for low rate operations and Li/Mn02 for high rate functions [38]. 

It is not possible to define experimentally the effects of lead on cognitive function (e.g. learning and memory), and effects on arousal and motivation can complicate the observed picture. The definition of acquisition may also represent performance, as is the case in studies that report group differences on the first day of acquisition. Some types of learning problems are more sensitive to lead-related disruption than others. Simple pattern discrimination and passive avoidance are not especially sensitive to lead. Functional impairment by lead appears to increase as the task complexity increases (e.g. from discrimination tasks to differential reinforcement of low rate operant responding). This suggests that the possible consequences of lead exposure will only be uncovered when experimental design becomes more sophisticated than at present. 

These batteries are recommended for low-rate operations and they are particularly suited for applications requiring long life under low drain or open-circuit conditions. Different cathodes have been used in these commercial solid-state cells, including a mixture of Pbl2 + Pb or Pbla + PbS + Pb, and a new system under development uses a mixture of TiS2 + S or AS2S3, which increases the energy density. The solid electrolyte is a dispersion of Lil and LiOH with alumina. Lithium... 

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