Membranes Rectifying section

Figure 4.2 Schematic representation of a membrane operation corresponding to the top section or rectifying section of a distillation column ...

As in distillation, sidestreams can be introduced or sidestream products can be withdrawn at different points or stages. In distillation practice at least, it is preferable that the sidestreams introduced be approximately of the same composition as at the point or stage introduced. Again by analogy, the membrane section above the feed location may be referred to as the rectifying section, and the section below as the stripping section or vice versa. The former is the accepted convention in distillation practice, where the more-volatile component(s) move toward the top of the column, the less-volatile toward the bottom. 

On the other hand, when applied to multistage operations, the concept of F pertains to the introduction of both V and L (that is, to V + L) to the reject side of the membrane, where V and L originate from the adjacent membrane cells, that is, from the posterior and anterior cells. Thus, the so-called internal reflux ratio or recycle ratio L/V can be used to establish a value for V/F, from which a value for the permeate flux V" could in turn be calculated by the same methods of Chapter 3. The value of V" so determined is the uniform and constant permeate rate at each stage in the rectifying section. This would supersede the determination of the permeate flux V" based on the feedstream per se. 

The corresponding spreadsheet calculations are furnished in Appendix 4, including the determination of membrane cell area, and these may be applied to other circumstances and specifications. More rigorously speaking, there is a relationship between the rectifying section and stripping sections, which, given LID or L/V and implies a relationship between V/B or L/V (or V/L] and as previously derived for X = 1. This, however, introduces an extra element of trial and error, a refinement not always warranted for estimation purposes. 

However, from this point on, a degree of separation is assigned in terms first of (Xj)), and then (Xg)., along with specifying the external recycle ratios L/D and V/B, and the necessary membrane areas are determined by analytic integration, for both the rectifying sections and stripping sections. 

Case 1 would represent, for example, the terminating sections (e.g., rectifying) of an entire membrane column configuration. Case 2, alternatively, would be a general MCS placed anywhere within the membrane column or cascade. 

Full plant shutdowns are necessary in bipolar cell rooms that contain only one electrolyzer. Since work must be carried out on the installed electrolyzer to change modules, electrodes, or membranes, downtime can be significant and is a function of the amount of work to be done and the design of the electrolyzer. Bipolar cell rooms with multiple electrolyzers may have one of several different configurations. Several electrolyzers may be fed in parallel from a common rectifier (common in mercurycell conversions), or individual electrolyzers may be equipped with their own rectifiers (technically, the best modem practice). Electrolyzers also may be made up of a number of packs or frames arranged in electrical series. Section 8.3.1.3 discussed some of these combinations. 

The number of membrane stages or cells is determined from a stepwise procedure, starting, say, from the more-permeable product D. It can be assumed that the recycle or reflux ratios for the rectifying and stripping sections have been specified, and the feedstream partitioning X, if any. The first few steps are illustrated in Figures 4.6 through 4.8. 

If the rectifying and stripping sections are linked together, then at the feed membrane cell, for a component /,... 

That is to say, the feedstream mixture is assumed injected into the reject side of the membrane cell at the feed location. This is the simplest embodiment, since V" does not change between the rectifying and stripping sections. It could as well be injected into the permeate side, however, or prorated between the reject and permeate sides. 

The same membrane characteristics and operating conditions are used as in the case of Example 4.1, which are the same as for Example 3.1. In Example 4.1, however, the /C-value concept was utilized to establish stagewise absorption and stripping factors for the rectifying and stripping sections, where for component /,... 

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