Stream Splitting

Stream splitting was originally exploited to improve the sampling step in segmented flow analysis for enzymatic assays or simultaneous determinations. 

As stream segmentation prior to stream splitting was not involved in either of these classical applications, the flow rates of the emergent streams were governed by the aspiration rates of a peristaltic pump. Although the splitting process is a simple concept, its initial development required much effort. After the experience with a two-channel system 

The sample is inserted into the reagent carrier stream which undergoes splitting, thus separating the sample zone into two portions (Fig. 7.5). For better sensitivity, the sample can be inserted into a chemically inert carrier stream, the reagent being added by confluence immediately after the insertion port (see 3.2.2). 

After splitting, the resulting streams are directed towards two parallel reactors at different flow rates, governed by the hydrodynamic pressures involved. Consequently, most of the original carrier stream (hence the 

FIGURE 7.5 Didactic representation of a flow analyser with stream splitting/stream merging. C = reagent carrier stream S — sample insertion R(1 and Rq = reaction coils x and y = splitting and merging points D — detector solid arrow = site where pumping is applied. For details, see text. 

The pinch design method developed earlier followed several rules and guidelines to allow design for minimum utility (or maximum energy recovery) in the minimum number of units. Occasionally, it appears not to be possible to create the appropriate matches because one or other of the design criteria cannot be satisfied. 

Consider Fig. 16.11a, which shows the above-pinch part of a design. Cold utility must not be used above the pinch, which means 

If there had been more cold streams than hot streams in the design above the pinch, this would not have created a problem, since hot utility can be used above the pinch. 

By contrast, now consider part of a design below the pinch (Fig. 16.12a). Here, hot utility must not be used, which means that all cold streams must be heated to pinch temperature by heat recovery. There are now three cold streams and two hot streams in Fig. 16.12a. Again, regardless of the CP values, one of the cold streams cannot be heated to pinch temperature without some violation of the constraint. The problem can only be resolved by splitting a hot (a) 

Sc = number of cold streams at the pinch (including branches) 

Example 18.2 A problem table analysis for part of a high-temperature process reveals that for A Tmi = 20° C the process requires 9.2 MW of hot utility, 6.4 MW of cold utility and the pinch is located at 520°C for hot streams and 500°C for cold streams. The process stream data are given in Table 18.2. Design a heat exchanger network for maximum energy recovery that features the minimum number of units. 

---

Figure 16.11 If the number of hot streams above the pinch at the pinch is greater than the number of cold streams, then stream splitting of the cold streams is required.

It is not only the stream number that creates the need to split streams at the pinch. Sometimes the CP inequality criteria [Eqs. (16.1) and (16.2)] CEmnot be met at the pinch without a stream split. Consider the above-pinch part of a problem in Fig. 16.13a. The number of hot streams is less than the number of cold, and hence Eq. (16.3) is satisfied. However, the CP inequality also must be satisfied, i.e., Eq. (16.1). Neither of the two cold streams has a large enough CP. The hot stream can be made smaller by splitting it into two parallel branches (Fig. 16.136). 

Clearly, in designs different from those in Figs. 16.13 and 16.14 when streams are split to satisfy the CP inequality, this might create a problem with the number of streams at the pinch such that Eqs. (16.3) and (16.4) are no longer satisfied. This would then require further stream splits to satisfy the stream number criterion. Figure 16.15 presents algorithms for the overall approach. ... 

Figure 16.13 The CP inequality rules can necessitate stream splitting above the pinch.

Thus loops, utility paths, and stream splits offer the degrees of freedom for manipulating the network cost. The problem is one of multivariable nonlinear optimization. The constraints are only those of feasible heat transfer positive temperature difference and nonnegative heat duty for each exchanger. Furthermore, if stream splits exist, then positive bremch flow rates are additional constraints. 

If the network is optimized at fixed energy consumption, then only loops and stream splits are used. When energy consumption is allowed to vary, utility paths also must he included. As the network energy consumption increases, the overall capital cost decreases. 

Figure B.l shows a pair of composite curves divided into vertical enthalpy intervals. Also shown in Fig. B.l is a heat exchanger network for one of the enthalpy intervals which will satisfy all the heating and cooling requirements. The network shown in Fig. B.l for the enthalpy interval is in grid diagram form. The network arrangement in Fig. B.l has been placed such that each match experiences the ATlm of the interval. The network also uses the minimum number of matches (S - 1). Such a network can be developed for any interval, providing each match within the interval (1) satisfies completely the enthalpy change of a strearh in the interval and (2) achieves the same ratio of CP values as exists between the composite curves (by stream splitting if necessary).

Fractionators produce two results only (1) stream splitting, with so many pounds going out one end and all other feed pounds going out the other and (2) component segregation toward one or the other of the product streams, characterized by the Fenske ratio ... 

The introduction of the sample valve, however, helped establish radial equilibrium early in the separation but, unless some special sample spreading device is employed at the front of the column, equilibrium will not necessarily occur at the point of injection. The stream splitting process is depicted in Figure 2. 

What is the optimal system configuration (e.g., how should these mass exchangers be arranged Is there any stream splitting and mixing ) ... 

Once again, stream splitting may be required to guarantee that inequality (5.12a) or (5.12b) is realized for each pinch match. It should also be emphasized that the feasibility criteria (Eqs. 5.8 and 5.12) should be fulfilled only at the pinch. Once... 

The combustion of mixtures of hydrogen and air produces very few ions so that with only the carrier gas and hydrogen burning an essentially constant signal is obtained. When, however, carbon-containing compounds are present ionisation occurs and there is a large increase in the electrical conductivity of the flame. Because the sample is destroyed in the flame a stream-splitting device is employed when further examination of the eluate is necessary this device is inserted between the column and detector and allows the bulk of the sample to by-pass the detector. 

Figure 18.15 The CP in equality rules can necessitate stream splitting below the pinch.

One further important point needs to be made regarding stream splitting. In Figure 18.14, the hot stream is split into two branches with CP values of 3 and 2 to satisfy the CP inequality criteria. However, a different split could have been chosen. For example, the split could have been into branch CP values of 4 and 1, or 2.5 and 2.5, or 2 and 3 (or any setting between 4 and 1, and 2 and 3). Each of these would also have satisfied the CP inequalities. Thus,... 

---