Two-tower system

The demand for isoprene for Butyl rubber led to the development of a recovery process for this Cj diolefin. Extractive distillation with acetone was the first process used but it has been replaced with acetonitrile (ACN ). The first step in the process is the fractionation of steam cracker debutanizer bottoms in a conventional two tower system to produce a C5 cut containing 30% isoprene. The first tower rejects C and heavier while the second rejects C4 and lighter materials. 

A schematic flow chart of this process is shown in Fig. 6. The feed is composed of 25-35 wt% purified isobutene and the required amount of isoprene (0.4-1.25 wt%), dissolved in cooled liquid methyl chloride. Purity of supplied isobutene is variable and preliminary drying and fractionation in a two-tower system remove water, n-alkenes, f-butanol, and diisobutenes. The polymerization grade (>99.5%) fresh isobutene is then mixed in a feed blend drum with isoprene (purity >98%) and a recycle stream of diluent and unreacted monomers. A concentrated catalyst solu-... 

This two-tower system can be used to separate a number of binary systems, such as isobutanol-water, aniline-water, or benzene-water. In the last system the solubility of water in benzene is so low that a single-tower system is usually used for the dehydration of benzene,... 

The benzene-alcohol-water system can produce an overhead vapor that will give two liquid phases on condensation which makes it possible to by-pass the azeotrope in a manner analogous to that which was shown for partly miscible binary distillations, and the same type of two-tower system is applicable. It should be noted that the system does not produce the ternary azeotrope as the overhead composition. 

For the purposes of this example it is assumed that the feed to the dehydration system contains 89 mol per cent alcohol and 11 mol per cent water A two-tower system will be employed similar to that illustrated in Fig. 10-12, and only a single liquid layer will be refluxed to each tower. Both towers will be designed for an overflow rate below the feed plate of 125 mols per 100 mols of vapor, and the usual simplifying assumptions will be made. The feed to the alcohol tower will be such that Vn Vm, and it is assumed that any condensation due to the reflux liquids... 

Fig. 1. The energy cycle of a thermal electric generating station having two alternative cooling systems (—) the open-circuit or once-through system and (-------------------------------------) a representative closed-cycle, cooling-tower system. Reproduced by permission (3).

Gas plants are integrated tower systems intended to recover LPG range material and separate it from naphtha products. This stabilizes the naphtha and reduces its vapor pressure. The LPG material may either be saturate gases going to LPG or unsaturates going to further processing. Gas plants on preflash and atmospheric crude processing units are saturate gas plants. Gas plants on FCC units are unsaturate gas plants. Coker and visbreaker gas plants are somewhere between the two. 

Figure 8-21 is a flow diagram for a typical two-tower solid desiccant dehydraiion unit. The essential components of any solid desiccant dehydration system are ... 

Two towers operate continuously in series at a steady state. The first, smaller, column treats the steady vents (e.g. tail gas) while the second, larger tower is designed to cope with emergency relief streams. The fresh caustic is fed to the downstream tower and the smaller tower make-up and blow-down are controlled by ORP measurement. The smaller tower operates at near exhaustion of chlorine while the second contains virtually fresh caustic. This system is ideally suited to the production of saleable, 15 wt% hypochlorite. 

So far, all our examples have dealt with two-component systems. And many of our towers really just have two components. Also, we have assumed that the reflux rate is large compared to the overhead product rate. And many of our towers do run with a lot of reflux. But we can all... 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

A petrochemical facility with one cooling system containing two towers, each installed at the same time, but with some differences in construction materials. Makeup water was 45% demineralized water and 55% raw water. 

Forced oxidation is achieved by air sparging of the slurry in an oxidation tank, either on the bleed stream to the solids dewatering system or on the recirculated slurry within the scrubber slurry loop. For a one-scrubber-loop forced oxidation system, the slurry effluent from all scrubbers in the system (e.g., the venturi scrubber and spray tower at Shawnee constitute a two-scrubber system, and the spray tower alone or TCA, a one-scrubber system) are sent to a single effluent hold tank, which is the oxidation tank. For a two-loop forced oxidation system, there are two scrubbers in series (e.g., venturi and spray tower at Shawnee) with effluent from each scrubber going to a separate tank the effluent hold tank for the upstream scrubber (with respect to gas flow) is the oxidation tank. For either one-loop or two-loop forced oxidation systems, the oxidation tank may be followed by a second tank, in series, to provide further limestone dissolution and gypsum desupersaturation time prior to recycle to the scrubber. 

Limestone Long-Term Tests with Two Scrubber Loops and Forced Oxidation. The venturi/spray tower system was modified for two-scrubber-loop operation with forced oxidation as shown in Figure 2. Two tanks were used in the oxidation loop (venturi loop) air was injected to the first of these tanks through a simple 3-inch diameter pipe below the agitator. Adipic acid was dry-fed to the spray tower effluent hold tank. This was accomplished by manually adding one-pound increments hourly to maintain specified concentration, usually totaling only a few pounds per hour. A small screw feeder would serve the purpose in a full-scale plant. 

Figure 2. Flow diagram for adipic acid-enhanced scrubbing in the venturi/spray tower system with two scrubber loops and forced oxidation.

Adipic Acid-Enhanced Limestone Tests on the Two-Loop Venturi/Spray Tower System with Forced Oxidation... 

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