Countercurrent flow limit

Hsieh and McNulty [210] developed a new correlation for weeping of sieve and valve trays based on experimental research and published data. For sieve trays the estimation of the weeping rate and weep point is recommended using a two-phase countercurrent flow limitation model, CCFL. 

There are a number of situations where countercurrent two-phase flow can exist in nuclear reactor coolant channels. For example, during emergency core cooling of the BWR rod bundles at low flow has steam and water countercurrent flow. The water flow rate can continue for certain ranges of water and steam flow rates. However, the relative velocity between the steam and water creates waves on the liquid surface for large gas velocities. And as the steam velocity increases the waves reach the channel walls and block the downward flow of the water. This transition point is called flooding or countercurrent flow limit. Further increase in steam velocity leads another transition where water is carried upward and thus flow reversal occurs. The transitions are associated with large pressure drop in the pipe. 

The ice crystals must be separated from the saline solution surrounding them, and washed with freshwater. This is accompHshed by a downward countercurrent flow of a small amount of freshwater through the ice slurry in the washer—melter unit. Keeping that unit at about 0°C limits the needed pressure rise by the compressor to only about 130—260 Pa, and an auxiUary refrigerator is often used to compensate for heat gains from the ambient and the compression. 

General Characteristics Countercurrent flow is maintained by virtue of the difference in densities of the hquids and either the force of gravity (vertical towers) or centrifugal force (centrifugal extractors). Only one of the liquids may be pumped through the equipment at any desired velocity. Tne maximum velocity for the second is then fixed if it is attempted to exceed this limit, the second hquid will be rejected and the extractor will be flooded. 

Countercurrent flow has advantages in product and thermodynamically limited reactions. Catalytic packings (see Figure 9. Id) are commonly used in that mode of operation in catalytic distillation. Esterification (methyl acetate, ethyl acetate, and butyl acetate), acetalization, etherification (MTBE), and ester hydrolysis (methyl acetate) were implemented on an industrial scale. 

In the limit, the defining model equations for countercurrent flow become... 

While the shell-and-tube heat exchanger is the most commonly used in the process industries, it has the disadvantages that the flow is not truly countercurrent, which limits the minimum temperature difference that can be accommodated, and the area density is relatively low. Commonly used alternatives for shell-and-tube heat exchangers are ... 

Bypassing-Controlled Trayed or packed columns operate with countercurrent flow and can achieve many equilibrium stages in series by good distribution of gas and liquid, and careful control of details. Other devices such as sprays are vulnerable to bypassing and are limited to one equilibrium stage. 

Countercurrent Flow The gas flows up countercurrent with the downflow liquid. This mode of operation is not as widely used for catalytic reactions since operation is limited by flooding at high gas velocity at flooding conditions increasing the liquid flow does not result in increase of the liquid holdup. 

To obtain a better idea of the latter limits, a special measure was taken to avoid the interaction of liquid and gas at the channel outlet viz., gas was introduced through a capillary inserted in the bottom end of the channel, and the liquid was drained through a wad of mineral wool, as shown in Fig. 14. Although this solution may not be of industrial interest, it is helpful in understanding the hydrodynamic behavior of countercurrent flow in the internally finned channel in the absence of inlet and outlet effects. 

In contrast to packed catalyst beds, however, countercurrent flow of gas and liquid is in principle possible in internally finned monoliths at realistic fluid velocities that are of interest for large-scale industrial applications. The main limitation to countercurrent flow at high velocities is at the outlet of a channel, rather than in the channel itself. With a suitable design of the outlet geometry, however, this problem can be alleviated so that countercurrent operation becomes possible in the velocity range of interest. 

---