Limitation point, countercurrent

In order to allow integration of countercurrent relations like Eq. (23-93), point values of the mass-transfer coefficients and eqiiilibrium data are needed, over ranges of partial pressure and liquid-phase compositions. The same data are needed for the design of stirred tank performance. Then the conditions vary with time instead of position. Because of limited solubihty, gas/liquid reactions in stirred tanks usually are operated in semibatch fashion, with the liquid phase charged at once, then the gas phase introduced gradually over a period of time. CSTR operation rarely is feasible with such systems. 

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. 

The premises have now been provided for calculating the operation limits (i.e., the highest possible flow rates of a countercurrent column). This is reached as soon as no further increase of the flow rate can be performed by the density difference. The consequence is that the light phase leaves the column at the bottom instead of at the top and the opposite situation for the heavy phase (see Fig. 9.1). This load limit is also called the flooding point. 

Recently Bashforth et al. (B15) have made a study of the special region in which the transition from countercurrent to cocurrent liquid flow occurs with vertical upward gas flow. At this point, the liquid film is suspended, and the net liquid flow rate is zero corresponding to the limiting flooding case for wetted wall columns. 

In the design of a hne-chemical plant the size of the equipment, especially the volume of the reaction vessels, is critical. In order to ensure that the potential customer s needs are met by the capabilities of the plant, this has to be dehned in close coordination with the marketing and sales function. Depending primarily on the differing quantities of the hne chemicals to be produced in the same multipurpose unit, the concentration of substances in the reaction mixture, and the reaction time, there is, however, an upper limit for the size of the reaction vessel and the ancillary equipment. Some factors run countercurrent to the economy of scale and point to small-size equipment ... 

Normally, the concentration of solute in the final extract is limited to the value in equilibrium with the feed, but a countercurrent stream that is richer than the feed is available for enrichment of the extract. This is essentially solvent-free extract as reflux. A flowsketch and nomenclature of such a process are given with Example 14.7. Now there are two operating points, one for above the feed and one for below. These points are located by the following procedure ... 

By calculating the class 1 FI target, the process engineer can identify the critical uncertainty point and critical constraint (appearance of new pinches, nonnegative heating or cooling, and so on). This uncertainty point and constraint limit the resilience of a completely countercurrent (e.g., infinitely cyclic) HEN structure able to mimic the composite curves thus they seem the most likely uncertainty point and constraint to limit the resilience of a practical but well-designed (almost completely countercurrent) HEN structure. 

The characteristic velocity k is a function of droplet size, density difference, viscosity, etc. Thus, the holdup tends to increase either as the superficial flow velocities Uc and Ud are increased or as the characteristic velocity is reduced (e.g., by increasing agitation). A point is eventually reached where the increase in holdup becomes unstable (typically when = 0.3-0.4). This phenomenon is known as flooding, and it imposes a limit on the flow rates and agitation levels that can be used in countercurrent extraction processes. 

Figure 2.2-9 Flooding point diagram for countercurrent gas-liquid flow [12] after Billet [13]. The lines mark the upper limit of gravity driven countercurrent flow. Under usually operating conditions, 80% of that limit can be used.

The limiting velocity is to be determined for the countercurrent distillation of a C fatty acid at 20 mm Hg (boiling point 125.0°C at 20 min), using 4 mm saddle packing. 

To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. 

The material and rate balances are negative or positive, depending on whether flow is concurrent or countercurrent. As set forth in Figure 6.1, the convention used is that the direction of integration is from one end of the cell to the other for each stream that is, the limits for V and L are at the same common ends of the cell, designated point 1 and point 2, albeit the flows are in opposite directions in the case of countercurrent flow. 

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. 

However, from a hydrodynamics point of view, this is very difficult to run. Primary reasons for this difiiculty comes from the fact that countercurrent systems always have flooding limitations, and the window of flow rates for stable operation is relatively small. Thus, the typical choice for stable operation of slurry reactors is co-current systems (Figure 6.3b), in which the gas and liquid is arranged to flow concurrently and the... 

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