Sidestream

If the feed is dominated by the middle product (typically more than 50 percent) and the lightest product is present in small quantities (typically less than 5 percent), then the arrangement shown in Fig. 5.106 can be an attractive option. This time the light product must find its way up the column past the sidestream. Again, unless the light product is a small flow and the middle product a high flow, a... 

Consider a three-product separation as in Fig. 5.11a in which the lightest and heaviest components are chosen to be the key separation in the first column. Two further columns are required to produce pure products (see Fig. 5.11a). However, note from Fig. 5.11a that the bottoms and overheads of the second and third columns are both pure B. Hence the second and third columns could simply be connected and product B taken as a sidestream (see Fig. 5.116). The arrangement in Fig. 5.116 is known as a prefractionator arrangement. Note that the first column in Fig. 5.116, the prefractionator, has a partial condenser to reduce the overall energy consumption. Comparing the prefractionator arrangement in Fig. 5.116 with the conventional... 

As shown in Fig. 10.6, the vapor from the reactor flows into the bottom of a distillation column, and high-purity dichloroethane is withdrawn as a sidestream several trays from the column top. The design shown in Fig. 10.6 is elegant in that the heat of reaction is conserved to run the separation and no washing of the reactor... 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

The first step in CTO distillation is depitching. A relatively small distillation column is used as a pitch stripper. The vapor from the pitch stripper is fed directiy into the rosin column, where rosin and fatty acids are separated. Rosin is taken from the bottoms of the column and fatty acids as a sidestream near the top. Palmitic acid and light neutrals are removed in the rosin column as heads. The operation is designed to minimize holdup and product decomposition. Care is taken to prevent carryover of some of the heavier neutrals, such as the sterols, from the depitcher to the rosin column (24). 

The rosin column spHt is controUed by the fatty acid content specified for rosin. This is usuaUy set at 2% fatty acids. At the high temperature near the bottom of the column and the reboUer, rosin dimerizes to some extent. By taking rosin from the column as a sidestream above the bottom, its rosin dimer content is minimized. Because of its high purity, sidestream rosin product is prone to crystallization. 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

L Liquid flow rate (kg-mol)/s (Ib-mol)Ai s Liquid-sidestream ratio ... 

S Vapor-sidestream ratio vapor that is not absorbed ... 

FIG. 13-6 Thermally coupled systems for separation into three products, (a) Fractionator with vapor sidestream and side-cut rectifier, (h) Petlyuk towers. 

A simple equilibrium stage (no feed or sidestreams) is depicted in Fig. 13-22. Four mass streams and a heat-leak (or heat-addition) stream provide the following number of variables ... 

The constant-molar-overflow assumption represents several prior assumptions. The most important one is equal molar heats of vaporization for the two components. The other assumptions are adiabatic operation (no heat leaks) and no heat of mixing or sensible heat effects. These assumptions are most closely approximated for close-boiling isomers. The result of these assumptions on the calculation method can be illustrated with Fig. 13-28, vdiich shows two material-balance envelopes cutting through the top section (above the top feed stream or sidestream) of the column. If L + i is assumed to be identical to L 1 in rate, then 9 and the component material balance... 

The effect of a sidestream withdrawal point is illustrated by Fig. 13-29. The material-balance equation for the column section below the sidestream is... 

FIG. 13-29 Material-balance envelope which contains two external streams D and S, where S represents a sidestream product withdrawn above the feed plate,... 

If the sidestream in Fig. 13-29 had been a feed, the balance for the section below the feed would be... 

Thermal Condition of the Feed The slope of the operating line changes whenever a feed stream or a sidestream is passea. To calculate this change, it is convenient to introduce a quantity q which is defined by the following equations for a feed stream F ... 

Equations analogous to (13-26) and (13-27) can be written for a sidestream, but the 9 will be either 1 or 0 depending upon whether the sidestream is taken from the hquid or the vapor stream. 

The derivation of Eq. (13-28) assumes a single-feed column and no sidestream. However, the same result is obtained for other column configurations. Typical 9-line constructions for sidestream stages are shown in Fig. 13-33. Note that the 9 hne for a sidestream must always intersect the diagonal at the composition (91 or Xi) of the sidestream. 

FIG. 13-33 Typical construction for a sidestream showing the intersection of the two operating lines with the q line and with the x — q diagonal, (a) Liquid sidestream near the top of the column, (h) Vapor sidestream near the bottom of the column. 

Another entire column with a partially vaporized feed, a hqnid-sidestream rate equal to D and withdrawn from the second stage from the top, and a total condenser is shown in Fig. 13-36. The specified concentrations are Xp = 0.40, Xb = 0.05, and Xo = 0.95. The specified L/V ratio in the top sec tion is 0.818. These specifications permit the top operating hne to be located and the two top stages stepped off to determine the liqnid-sidestream composition Xs = 0.746. The operating line below the sidestream must intersect the diagonal at the blend of the sidestream and the overhead stream. Since S was specified to be equal to D in rate, the intersection point is... 

This point phis the point of intersection of the two operating lines on the sidestream q line (vertical at Xs = 0.746) permits the location of the middle operating line. (The slope of the middle operating line conld... 

In addition, sidestream flow rates are replaced with sidestream flow ratios by... 

Miiltiple feeds, sidestreams, and/or intermediate condensers and/or intermediate reboilers are required or desirable. 

In contrast to segregated flow, in which the mixing occurs only after each sidestream leaves the vessel, under maximum mixedness mixing of all molecules having a certain period remaining in the vessel (the life expectation) occurs at the time of introduction of fresh material. These two mixing extremes—as late as possible and as soon as possible, both consistent with the same RTD—correspond to performance extremes of the vessel as a chemical reactor. 

Figure 4-13. Sidestream radial machines of four to eight stages with horizontally split casing and one or two pairs of intermediate nozzles, generally for connecting external intercoolers.

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