Exchange columns, cascade

Figure 3. Exchange column, thermal refluxer, and reflux reactor for the NO-N2O3 solvent carrier system. The parts of the system are discussed in the text. X and Y indicate the positions to which connections are made when two columns are operated in cascade...

Table VIII. Summary of the Characteristics and Operating Conditions for the Cascade of Two Exchange Columns for Preparing 99.5% Nitrogen-15 with the NO-NjO Solvent Carrier System...

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

This uncatalyzed reaction occurs at an acceptable rate at room temperature (298 K), a = (15/14)aq/(15/14)gas = 1.055. The value calculated from spectroscopic data is 1.096 (Chapter 4), so parasitic bleed to species other than NO(g) and HN03(aq) is indicated (the chemistry shown in Equation8.13 is oversimplified). In the exchange aqueous acid trickles down through a packed column countercurrent to a rising NO stream. A two column cascade was employed in which 4% of the acid flow is... 

Computer simulation models have been formulated for cascade and Stratco sulfuric acid alkylation units. These complete models incorporate mathematical descriptions of all the interacting parts of the units, including reactors, distillation columns, compressors, condensers, and heat exchangers. Examples illus-strate diverse model applications. These Include identifying profitable unit modifications, comparing cascade to Stratco performance, evaluating optimal unit capacity and determining optimal deisobutanizer operation. 

The great advantage of the described condensation resin with [2b.2.2] anchor groups lies in the very favourable isotopic separation dependent on the eluted amount of substance (see Fig. 16) and by the fact that a pure solvent can be used as eluant which simplifies the isolation of the isotopic-enriched fractions. Now, additional experiments with larger columns and the application of the cascade principle (Chap. 2.5.2) will have to prove whether or not an enrichment of the heavy isotopes on a technical scale is possible with this exchange system. 

Figure 20.3 Examples of cascade control for (a) heat exchanger (b), (c) distillation column (d) process furnace.

The cascade shown in Figure 3-5 can be further simplified by building the entire system in a column instead of as a series of individual stages. The intermediate heat exchange can be done very efficiendy with the liquid and vapor in direct contact on each stage. The result is a much sinpler and cheaper device. A schematic of such a distillation column is shown in Figure 3-6. 

For traditional cascaded plants, consisting mainly of process units connected in series, the controllability properties can easily be deduced from consideration of the individual units. The disturbance sensitivity is simply a product of the disturbance sensitivities of the units cormecting the disturbance and output under consideration, and any fundamental control limitation can be attributed to some individual unit. Since there exist a wealth of knowledge on how to improve the disturbance sensitivity, and avoid control limitations, through design of common units like reactors, distillation columns and heat-exchangers, design for controllability is fully feasible for cascaded plants. 

In the oxidation of cyclohexane, the efficiency is relatively low, which is the reason why research into and optimization of this process step was taken in hand. The flowsheet of the oxidation section is shown in fig. 1. Fresh cyclohexane is mixed with the recycled cyclohexane from the top of the cyclohexane distillation column. The mixture is fed to the condensation section to exchange heat with the vapour stream from the reactors. Next, the cyclohexane is oxidized with air in a cascade of stirred reactors. The oxidate leaving the reactors undergoes an after-treatment in a decomposition reactor, while the acids formed as byproduct in the oxidation section are neutralized in a mixer-settler unit. In the cyclohexane distillation section non-converted cyclohexane is separated from cyclohexanol, cyclohexanone and byproducts. 

FIGURE 11.11 Flowsheet of the cascade process of the partial oxidation of natural gas [274]. (1) Gas pipeline, (2) compressor (3) recuperative heat exchanger, (4) air compressor, (5) reactor (oxidation stage), (6) controls of steam parameters, (7) heat exchanger, (8) separator, (9) throttle, (10) receptacle of liquid products, and (11) rectUying column. 

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