Cascade countercurrent recycle

The countercurrent (recycle) cascade (Fig. 8.3) is much more useful because by reprocessing the waste stream a larger fraction of the desired isotope is recovered. In a recycle cascade the i th stage is fed by a mixture of the product, Y( i) from the (i — l) th, and the waste, X(i+ X) from the (i + l) th, stage. A distillation column... 

When partially depleted tails have sufficient value to warrant reprocessing, a countercurrent recycle cascade like Fig. 12.13 may be used. This cascade flow scheme is by far the most... 

Separative stages arranged in a countercurrent recycle cascade... 

In Table 8.6, the results of computations identical in principle to those described previously are given. The final extract from the countercurrent cascade described in Table 8.5 was contacted with a pure stream of the more extractable component at a high phase ratio. The high phase ratio is chosen to minimize the volume of aqueous phase that must be recycled to the feed. 

The product recovery for a countercurrent cascade is much better than for the series of flash units without recycle. First, we have only one waste stream (5) — instead of one from each flash. Second, the composition of this waste stream will usually turn out to be leaner in the valuable component - because we are enriching the feed to the first flash by recycling from upstream. 

A flow diagram for countercurrent extraction with reflux is shown in Fig. 20.14. To emphasize the analogy between this method and fractionation, it is assumed that the cascade is a plate column. Any other kind of cascade, however, may be used. The method requires that sufficient solvent be removed from the extract leaving the cascade to form a raffinate, part of which is returned to the cascade as reflux, the remainder being withdrawn from the plant as a product. Raffinate is withdrawn from the cascade as bottoms product, and fresh solvent is admitted directly to the bottom of the cascade. None of the bottom raffinate needs to be returned as reflux, for the number of stages required is the same whether or not any of the raffinate is recycled to the bottom of the cascade. The situation is not the same as in continuous distillation, in which part of the bottoms must be vaporized to supply heat to the column. 

Example 10.3. As shown in Fig. 10.22, a countercurrent extraction cascade equipped with a solvent separator to provide extract reflux is used to separate methylcyclopentane A and n-hexane C into a final extract and raffinate containing 95wt% and 5wt% A, respectively. The feed rate is 1000 kg/hr with 55 wt% A, and the mass ratio of aniline, the solvent S, to feed is 4.0. The feed contains no aniline and the fresh solvent is pure. Recycle solvent is also assumed pure. Determine the reflux ratio and number of stages. Equilibrium data at column temperature and pressure are shown in Fig. 10.23. Feed is to enter at the optimum stage. 

Direct continuous reaction of SOs with benzene, highly successful in the case of dimethyl ether as described above, is not practical because of high sulfone formation. Indirect continuous reaction with SOs by a procedure stated to yield no sulfone has, however, been achieved by the method developed by Dennis and Bull. This process is based upon an observation made by the former that, in the presence of sulfuric acid, benzene will dissolve 2-3 per cent of its own volume of benzenesulfonic acid. This process is also designed to operate in continuous countercurrent flow in a cascade system, benzene being introduced at the bottom and a benzene solution of the sulfonic acid overflowing from the top. Concentrated sulfuric acid is added continuously at the top, and spent sulfuric acid (77 per cent) is removed at the bottom of the reactor. The spent acid may be fortified to original strength with SO3 for reuse, and the benzene is recycled after the product sulfonic acid has been extracted from it with water. This procedure is, in theory, the most efficient possible, since benzenesulfonic acid is, in... 

Here we understand by complex colunms a countercurrent cascade without branching of flows, without recycles and bypasses, which, in contrast to simple columns, contains more than two sections. The complex colunm is a column with several inputs and/or outputs of flows. The column of extractive distillation with two inputs of flows - feed input and entrainer input - is an example of a complex column. 

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Cascading or countercurrent water-use systems (where an operation that requires relatively low quality water uses wastewater from another operation) or recycling systems (where water is treated and returned to the same operation) can greatly reduce intake water requirements. 

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