Countercurrent processes multiple stages

The need for a continuous countercurrent process arises because the selectivity of available adsorbents in a number of commercially important separations is not high. In the -xylene system, for instance, if the Hquid around the adsorbent particles contains 1% -xylene, the Hquid in the pores contains about 2% xylene at equiHbrium. Therefore, one stage of contacting cannot provide a good separation, and multistage contacting must be provided in the same way that multiple trays are required in fractionating materials with relatively low volatiHties. 

Now consider a multiple-stage process with countercurrent flow of the raffinate and extract phases. Fig. 2B. Countercurrent is the most efficient multistage configuration. One can write a mass balance around the nth stage, as indicated by envelope 1, using the following units for a continuous flowing process ... 

Absorber and strippers may be classified as complex columns because they possess two feeds and because they possess neither an overhead condenser nor a reboiler. The sketch of the absorber in Fig. 4-1 depicts an historic application of absorbers in the natural gas industry. From a light gas stream such as natural gas that contains primarily methane plus small quantities of, say, ethane through n-pentane, the desired quantities of the components heavier than methane may be removed by contacting the natural gas stream with a heavy oil stream (say n-octane or heavier) in a countercurrent, multiple-stage column such as the one shown in Fig. 4-1. Since absorption is a heat-liberating process, the lean oil is customarily introduced at a temperature below the average temperature at which the column is expected to operate. The flow rate of the lean oil is denoted by L0, and the lean oil enters at the top of the column as implied by Fig. 4-1. The rich gas (which is sometimes called the wet gas) enters at the bottom of the... 

SFE processes can be carried out in different modes single stage, multiple stage, multiple stage and countercurrently, as a chromatographic process, and with the aid of chemical reactions. 

The various process modes are applied primarily on the basis of the properties of the feed mixture. Solids are processed mainly in single stage or multiple stage operational mode. Fluid feed mixtures containing compounds of similar solubility in the solvent are best treated in a multiple stage countercurrent process. The separation of isomers falls within the field of chromatography or selectively catalyzed reactions. Chemical reactions, e.g. esterification of free fatty acids, may be part of the separation process. 

Figure 10.3-3. Number of stages in a countercurrent multiple-stage contact process.

Graphical determination of the number of trays. A plot of the operating-line equation (10.6-2) as y versus x will give a curved line. If x and y are very dilute, the denominators 1 — X and 1 — y will be close to 1.0, and the line will be approximately straight, with a slope L/V. The number of theoretical trays are determined by simply stepping off the number of trays, as done in Fig. 10.3-3 for a countercurrent multiple-stage process. 

It is proposed to reduce the concentration of acetaldehyde in aqueous solution from 50 per cent to 5 per cent by mass, by extraction with solvent S at 293 K. If a countercurrent multiple-contact process is adopted and 0.025 kg/s of the solution is treated with an equal quantity of solvent, determine the number of theoretical stages required and the mass flowrate and concentration of the extract from the first stage. 

The feed (H2O) is introduced at the top of a cold tower where it equilibrates in a countercurrent multiplate column against a gas stream (mostly H2S). The D concentration builds toward its maximum at the bottom of the cold tower. The essential distinction between the GS process and standard chemical reflux is that in the GS process, the reflux is carried out thermally. The hot tower serves as refluxer for the cold tower. At the top of the cold tower, an intermediate point in the plant, the D content of the gas stream, Ug, is set by equilibration against the cold feed, the separation factor is = [zf/(l Zf)]/[t (l — i )]. Next, that gas is introduced to the bottom of the hot tower where it equilibrates with the waste flow, aj, = xj (1 — x )]/[t /(l — t )]. (The symbols have been defined in earher sections.) The overall separation, S, in a stage containing both a hot and a cold tower is S = a.Ja.. Notice that S is an effective separation factor. 

Typically, the input flow of UFs is adjusted so that roughly half of the feed diffuses through the walls of the barrier tubes into interstitial space, becoming enriched (product), while the rest remains in the tubes, becoming depleted (tails). To obtain significant enrichments, this process must be repeated multiple times (Krass et al. 1983). In a cascade of diffusion cells, the product of one enrichment cell is used as feedstock for another. The cascade is simple if the tails are discarded the cascade is countercurrent if the tails are reintroduced as feed in a lower emichment stage. Simple cascades are not used because of the potentially profligate waste of uranium. After the initial start up of a countercurrent cascade, feed material is introduced only in the amount necessary to balance the withdrawal of product and tails. 

Let us illustrate how we can solve for ideal stages in an extraction system. The process that we will consider (see Figure 13-13) will be for a countercurrent multiple contact system with both extract and raffinate reflux. 

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