Absorption stage efficiency

The actual stage can be a mixing vessel, as in a mixer-settler used for solvent extraction applications, or a plate of a distillation or gas absorption column. In order to allow for non-ideal conditions in which the compositions of the two exit streams do not achieve full equilibrium, an actual number of stages can be related to the number of theoretical stages, via the use of a stage-efficiency factor. 

Sieve plates are used, similar to those used for distillation and absorption. The stage efficiency for sieve plates, expressed in terms the height of an equivalent theoretical stage (HETS), will, typically, range from 1 to 2.5 m. 

The methods based on the equilibrium stage model have existed for over 30 years and refinements continue, but serious development of nonequilibrium models has begun only recently. These methods are an alternative means to the stage model for predicting column performance. They are expected to make inroads, especially for systems for which stage efficiency prediction is very difficult, such as reactive distillation, chemical absorption, and three-phase distillation. However, their progress into systems where efficiency prediction is well-established is likely to be slower. Their complexity due to the restriction to... 

The small mismatch between required and achieved minimum operation temperature has the severe consequence that a special preabsorption stage has to be included in the reactor set-up in order to achieve the essential complete conversion. In this manner the partial pressure of the SO3 product is lowered before the last stage of conversion, rendering acceptable incomplete conversion of the overheated catalyst. If the reason why the catalyst docs not operate efficiently down to its solidification point could be eliminated one may circumvent the intermediate absorption stage and thus facilitate the reactor design considerably. 

Since most continuous extraction methods use countercurrent contacts between two phases, one a light liquid and the other a heavier one, many of the fundamentals of countercurrent gas absorption and of rectification carry over into the study of liquid extraction. Thus questions about ideal stages, stage efficiency, minimum ratio between the two streams, and size of equipment have the same importance in extraction as in distillation. 

The efficiency of a stage or plate in a distillation, absorption, or extraction operation is a function of the mass-transfer rates and transfer coefficients. When material is removed from a permeable solid, as in leaching or drying operations, the transfer rates and sometimes the stage efficiencies can be estimated from diffusion theory. 

In Fig. 9.4-3 the three adsorbent streams are bronght together and mixed with the result that they have the loading X. This loading can be found by the rule of balances or the mixing rale This rale is at first applied on the mass streams 5ri and S,2 and then on the binary mixture (4i + r2) and S s. The approach to equilibrium in a stage can be expressed by a stage efficiency, compare the sections on absorption, extraction, and rectification. This efficiency is mainly dependent on the residence time in relation to a characteristic diffusion time, see later. 

The mass transfer between gas and liquid phases on a column tray consisting of two phases is usually expressed by the enrichment ratio, plate efficiency E (see Chapter 1.1, stage efficiency factor). Since the main resistance to the mass transfer occurs in the gas phase during rectification and absorption, E is mainly defined as the efficiency with respect to gas. Eg or Eg . 

We have seen in Section 8.1.3 that multistage distillation is generally carried out in a vertical multiplate column in which each plate has crossflow. Vapor bubbles rise vertically through a liquid layer flowing horizontally in cross-flow over the plate (Section 8.1.3.5) the same two-phase flow scheme is employed in a plate for gas-liquid absorption stripping. We were introduced to the notion of stage efficiency, plate efficiency, tray efficiency, etc., in Section 8.I.3.4. We will introduce here the models used to... 

The absence of a concentration effect was also observed by Tohata et al. (1964A, B) for wetted wall and perforated plate columns. In the wetted wall column study, lithium chloride solutions in the range of 18.7 to 28.4% were employed. The results showed the gas-phase resistance to be controlling. Very high-stage efficiencies (up to 90%) were observed in the perforated plate column study, also indicative of a gas-phase resistance controlled absorption. 

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