Chemical reflux

The deuterium exchange reactions in the H2S/H2O process (the GS process) occur in the tiquid phase without the necessity for a catalyst. The dual-temperature feature of the process is illustrated in Figure la. Dual-temperature operation avoids the necessity for an expensive chemical reflux operation that is essential in a single-temperature process (11,163) (Fig. lb). 

Because of the very large enrichments required in heavy water production, cascades taper markedly. In the upper stages the relative advantage of chemical exchange over water distillation vanishes. Most heavy water plants carry out the last portion of the enrichment by distillation (from 20% or 30% D to 99.85%). Accordingly both exchange and distillation will be briefly treated below. First, however, to clarify the important distinction between chemical and thermal reflux we treat an example of isotope separation using chemical reflux. 

An Aside Monothermal Isotope Exchange with Chemical Reflux 15 N Enrichment... 

To avoid the high cost of chemical reflux the dual temperature H2S/H20 exchange was independently suggested by Geib (1946) and Spevack (1957) (GS). The method exploits the fact that the equilibrium constant for isotope exchange is temperature dependent. The scheme is illustrated in Fig. 8.13. To carry out the exchange... 

The utility and simplicity of a laboratory scale chemical exchange process with chemical reflux is illustrated by the Nitrox process for the production of (23). The exchange reaction is... 

Chemical reflux of a chemical exchange reaction accomplishes reflux by chemical inter-conversion of the two species. The conversion process supplies a countercurrent stream of enriched or depleted isotope of the appropriate isotopic composition. The use of a hot tower leads to a back transfer of enriched isotope from the enriching phase to the phase being depleted in the cold tower. The hot tower requires a number of plates comparable with that in the cold tower. The effective separation factor is, therefore. 

Despite these advantages of the water-hydrogen sulfide deuterium exchange reaction, it is not economical to use it in a monothermal flow sheet to produce heavy water because of the high cost of chemical reflux in this system. This may be shown by reference to Fig. 13.24. 

However, the cost of providing chemical reflux is so high as to preclude the use of the flow sheet of Fig. 13.24 for heavy-water production. From the preceding chemical reactions it is seen that mol of aluminum metal is consumed for each mole of D2S reflux. Because aluminum metal costs around S0.50/lb, the minimum cost of aiuminum (MW = 27) per pound of heavy-water product (MW = 20) is... 

The feed (H2O) is introduced at the top of a cold tower where it equilibrates in a countercurrent multiplate column against a gas stream (mostly H2S). The D concentration builds toward its maximum at the bottom of the cold tower. The essential distinction between the GS process and standard chemical reflux is that in the GS process, the reflux is carried out thermally. The hot tower serves as refluxer for the cold tower. At the top of the cold tower, an intermediate point in the plant, the D content of the gas stream, Ug, is set by equilibration against the cold feed, the separation factor is = [zf/(l Zf)]/[t (l — i )]. Next, that gas is introduced to the bottom of the hot tower where it equilibrates with the waste flow, aj, = xj (1 — x )]/[t /(l — t )]. (The symbols have been defined in earher sections.) The overall separation, S, in a stage containing both a hot and a cold tower is S = a.Ja.. Notice that S is an effective separation factor. 

METHOD 2 [89]--1M MDA or benzedrine and 1M benzaldehyde is dissolved in 95% ethanol (Everclear), stirred, the solvent removed by distillation then the oil vacuum distilled to give 95% yellow oil which is a Schiff base intermediate. 1M of this intermediate, plus 1M iodomethane, is sealed in a pipe bomb that s dumped in boiling water for 5 hours giving an orangy-red heavy oil. The oil is taken up in methanol, 1/8 its volume of dH20 is added and the solution refluxed for 30 minutes. Next, an equal volume of water is added and the whole solution boiled openly until no more odor of benzaldehyde is detected (smells like almond extract). The solution is acidified with acetic acid, washed with ether (discard ether), the MDMA or meth freebase liberated with NaOH and extracted with ether to afford a yield of 90% for meth and 65% for MDMA. That s not a bad conversion but what s with having to use benzaldehyde (a List chemical) Strike wonders if another aldehyde can substitute. 

Precondensate—HHj Process. From a historical point of view, the precondensate—ammonia process is a simplification of the THPC—amide—NH process. In this case, the chemical manufacturer forms a precondensate of THPC and urea by refluxing these components for about... 

Anhydrous hydrazine, required for propellant appHcations and some chemical syntheses, is made by breaking the hydrazine—water azeotrope with aniline. The bottom stream from the hydrate column (Fig. 4) is fed along with aniline to the azeotrope column. The overhead aniline—water vapor condenses and phase separates. The lower aniline layer returns to the column as reflux. The water layer, contaminated with a small amount of aniline and hydrazine, flows to a biological treatment pond. The bottoms from the azeotrope column consist of aniline and hydrazine. These are separated in the final hydrazine column to give an anhydrous overhead the aniline from the bottom is recycled to the azeotrope column. 

By fai the largest (ca 85% of the total) volume chemical blowing agent is azodicaibonamide (44), made by the oxidation of hydiazodicaiboxamide [110-21 -4] (51) using chlorine or sodium chlorate. The hydrazo precursor is made by refluxing an aqueous solution of urea and hydrazine (172) ... 

Checking Against Optimum Design. This attempts to answer the question whether a balance needs to be as it is. The first thing to compare against is the best current practice. Information is available ia the Hterature (13) for large-volume chemicals such as NH, CH OH, urea, and ethylene. The second step is to look for obvious violations of good practice on iadividual pieces of equipment. Examples of violations are stack temperatures > 150° C process streams > 120° C, cooled by air or water process streams > 65° C, heated by steam t/ urbine 65% reflux ratio > 1.15 times minimum and excess air > 10% on clean fuels. 

Partially Reversible Processes. In a partially reversible type of process, exemplified by chemical exchange, the reflux system is generally derived from a chemical process and involves the consumption of chemicals needed to transfer the components from the upflow into the downflow at the top of the cascade, and to accomplish the reverse at the bottom. Therefore, although the separation process itself may be reversible, the entire process is not, if the reflux is not accompHshed reversibly. 

Insofar as the consumption of chemicals is concerned, it is obvious that the total consumption of reflux-producing chemicals is proportional to the interstage flows, or width of the cascade, but independent of the number of stages in series, or length of the system. 

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