Cascades enriching section

Figure 6.16 Graphical evaluation of the number of stages for a reflux cascade (enriching section), operated with constant reflux.

Total Upflow in an Ideal Plant. The sum of the upflows from all of the stages in the ideal plant, or more simply, the total upflow, is the area enclosed by the cascade shown in Figure 4. An analytical expression for this quantity is obtained as the summation of all the stage upflows in the enriching section expressed as an integral ... 

Fig. 5. Design of a real cascade obtained by squaring off an ideal enriching section.

The Talcher HWP is coupled with a 900Mg/day ammonia synthesis plant of the Fertilizer Corporation of India to produce 62.5 Mg D20/yr. After purification, syngas enters the enrichment section, which consists of a three-stage cascade. The first stage is superimposed by two cold- and hot-stripping columns. Each stage consists of one pair of cold and hot columns. Deuterium concentration in ammonia is high... 

The streams that move away from the ends of the cascade, that is, the tails stream in the enriching section and the heads stream in the stripping section, are known as reflux. 

The total inventory of the enriching section Ip then is just h times the total flow rate in the enriching section for a close-separation, ideal cascade. 

In the enriching section of a cascade with constant tails flow rate N, the change in composition x with stage number i is given by differential equation (12.128). The number of... 

Equations for interstage flow rates and compositions in the enriching section are obtained by applying to the section of the cascade from the product end through stage / + 1 shown in Fig. 12.28 a development similar to the one used earlier for the complete cascade. 

How many stages are needed in the stripping section In the enriching section Where in the cascade does the maximum value of the reflux ratio (tails to product) occur What is this maximum value ... 

The total amount of hydrogen formed from stages m to n is given by (12.119), for the total tails flow in the enriching section of an ideal cascade, with x i replacing zp. In the plant shown in Fig. 13.16, the total number of moles formed is... 

Heads flow rate. The flow rate M of stage heads of composition y in the enriching section of a close-separation ideal cascade producing product at rate P and composition yp is... 

The slopes of the reflux lines may be either greater or less than those of the equilibrium curves, and by adjusting the reflux ratios at either end of the cascade it is possible to make stripping or enriching sections of either half of the cascade for either component, B and C. Only a detailed study of the equilibria for each system separately can establish the most desirable reflux ratios to be used, or indeed whether reflux is at all desirable. 

Gas extraction extends the possibilities of separation processes like distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. Furthermore, it enables separation of components with very similar properties if used in the countercurrent mode. Process temperatures in gas extraction are determined by the critical temperature of the solvent and not, as is the case of distillation of any kind, by the liquid-vapor transition of the feed mixture. As compared to liquid-liquid extraction, gas extraction makes easily possible to operate a two cascade separation column, applying a stripping and an enriching section. Combined, these possibilites allow gas extraction to be operated at very moderate temperatures and as a separation process for difficult separations. 

The feed to be fractionated, which can be a liquid, a vapor, or even a combination of both, enters the column at some central location rather tiian at one of the ends of the cascade as had previously been the case. This results in a division of the column into two parts, the rectification or enriching section above the feed tray, and the stripping or exhausting section below it. The upper section serves to enrich the vapor in the more volatile components, a portion of which is ultimately withdrawn as liquid "overhead product" or "distillate." In the lower section, residual volatile components are progressively stripped off the liquid and conveyed upward as vapor, while the downward flow of liquid becomes enriched in fhe heavier or less-volatile components. 

An arrangement for this is shown in Fig. 10.27. The feed to be separated into its components is introduced at an appropriate place into the cascade, through which extract and raffinate liquids are passing countercurrently. The concentration of solute C is increased in the extract-enriching section by countercurrent contact with a raffinate liquid rich in C. This is provided by removing the solvent from extract to produce the solvent-free stream E part of which is removed as extract product and part returned as reflux Rq. The raffinate-stripping section of the cascade is the same as the countercurrent extractor of Fig. 10.18, and C is stripped from the raffinate by countercurrent contact with solvent. 

These basic results allow us to determine the number of stages needed in the stripping section and the enriching section to achieve a certain difference in the composition from the top product Xiu to the bottoms product Xi26 in the ideal cascade having a feed of composition xy. For the enriching section, we can start from stage 1 ... 

The flow rate ratio Kin/Kin) called the internal reflux ratio, as opposed to external reflux ratio of definition (8.1.137) since no formal reflux stream is generated in Figure 9.1.1(b). This flow rate ratio provides an estimate of the heavy fraction flow rate downward in the cascade below stage n in the enriching section. We are interested in its variation with the stage number n. 

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