Double-entry separator

Chapter 2 presents the description of quantities needed to quantify separation in open systems with flow(s) in and out of single-entry and double-entry separators for binary, multi-component and continuous cheimcal mixtmes, as well as a size-distributed particle populatioiL Separation indices useful for describing separation in open systems with or without recycle or reflux are illustrated for steady state operation (Sections 2.2 and 2.3) those for a particle population are provided in Section 2.4. At the end (Section 2.5), indices for description of separation in time-dependent systems, e.g. chromatography, have been introduced. 

Open separators can be broadly classified into single-entry and double-entry separators. A single feed stream enters the sin -entry separator. Two feed streams are necessary far a double-entry separator (Figure 2.1.1). A feed stream enters a separator through a defined fraction of its surface area, just as the product streams (at least two in number in... 

Figure 2.1.1. Single-entry and double-entry separator. The velocity profiles shown at the tnlet(s) and exits of the separators need not he parabolic.

An open separator will have at least three streams coming in and out through three different surface areas Sj (see Section 2.5 for a different case). We adopt the convention that for a single-entry separator the subscript j will have values 1,2,..., k corresponding to k product streams, but j = /for the single feed stream. For a double-entry separator, the two feed streams are to be denoted hyj =/i and/2, respectively, while the k product streams will continue to have 7 = 1,2,..., k. 

With identical assumptions, the total mass balance and the ith species balance equations for a double-entry separator (see Figure 2.1.1) are ... 

The double-entry separator performance is often characterized for a two-component system by the equilibrium ratio Ki (definition (2.2. If)) as well as by the separation factor ajj (definition (2.2.2a)). Both definitions relate the two product stream compositions and are related to each other by (2.2.2i). The compositional differences between the light fraction and feed stream 1 may be indicated by a heads separation factor... 

Figure 2.3.1. Double-entry separator various orientations of feed streams.

The index extent of separation for a double-entry separator may be expressed as (Sirkar, 1977)... 

Figure 2.3.2. Packed column distillation of benzene-toluene mixture (a) double-entry separator (b) overhead condenser providing...

Example 2.3.1 described separation in a double-entry separator having two distinct phases - a vapor feed and a liquid feed, with both phases existing throughout the column. Double-entry separators are used even when two product streams and the two feed streams are of the same phase. 

The wall of the silicone capillaries separates the two gas streams in the permeator described above. It is not necessary to have a barrier between two streams within a separator. Later in the book we will come across double-entry separators in which two gas streams are in contact with each other and yet separation is taking place. 

Separation in a double-entry separator with recycle/reflux... 

We now briefly carry out a quantitative description of separation in a double-entry separator with recycle. A perfectly general double-entry separator with recycle would have two independent feed streams, and a part of one (or both) of the product streams is recycled to the feed stream closest to it (Figure 2.3.4). A more commonly encountered scheme would have only one independent feed stream entering the separator W = 0). For such a case, the total and the ith component molar balances are given by... 

Figure 2.3.4. Double-entry separator with or without reflux...

Figure 9.1.1. (a) Countercurrent cascade of double-entry separating elements as in distillation (e.g). (b) Countercurrent cascade of singleentry separating elements, (c) Compositions of various streams for stages (n - Ij, n and (n -y 1) and separation factors. 

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