Dual temperature process

Chemical exchange between hydrogen and steam (catalyzed by nickel—chromia, platinum, or supported nickel catalysts) has served as a pre-enrichment step in an electrolytic separation plant (10,70). If the exchange could be operated as a dual-temperature process, it very likely... 

Dual Temperature Process. A unit of a dual temperature cascade is shown schematically in Figure 8. The dual temperature system operates on the principle that isotope exchange reactions, like all chemical reactions, change their equilibrium constants with temperature. The general, but far from universal (22), rule is that in systems with large isotopic... 

Figure 13.27 Nomenclature for simpUHed treatment of dual-temperature process.

Figure 13.28 Example of McCabe-Thiele diagram for dual-temperature process.

Availability of this catalyst has led to interest in its possible use in dual-temperature water-hydrogen exchange processes. With liquid-water feed and recirculated hydrogen gas, this catalyst could be used in a dual-temperature process similar in principal to the GS process, with a schematic flow sheet like Fig. 1325. With ammonia synthesis-gas feed and recirculated water, this catalyst could be used in a dual-temperature process similar to the ammonia-hydrogen process flow scheme of Fig. 13.37, provided that impurities in synthesis-gas feed that would poison the catalyst can be recovered sufficiently completely. 

Miller and Rae [M7] have suggested process conditions for a dual-temperature process using this catalyst at 69 atm pressure and temperatures of S0°C for the cold tower and HO C for the hot. These conditions have been used to estimate optimum flow rates and numbers of theoretical stages for dual-temperature water-hydrogen processes using these two flow schemes. The results are tabulated in Table 13.28 and compared with similar data for the other dual-temperature processes discussed previously. 

In parallel with this, dual temperature processes were described by Rideal and Hartley in 1942. The earliest chemical engineering contributions therefore were made by men who would at that timie have described themselves as applied chemists but would, in their later careers, describe themselves as chemical engineers. During the period 1941-43 they had the advantage of exchange of information with the United States, until the latter withdrew their earlier cooperation. 

To avoid the high cost of chemical reflux, Geib (1946) and Spevack (1957) independently suggested a dual temperature process. Consider application to the H2S/H2O exchange known as the GS (Geib-Spevack, Girdler-Sulfide) process. The method exploits the fact that the isotope-exchange equilibrium constant is temperature-dependent. The scheme which results is illustrated in Fig. 51.7. Here the isotope of interest, D, concentrates in the liquid. 

The dual temperature process is based on the atomic exchange of hydrogen and deuterium between hydrogen sulfide gas (H2S) and fresh water with a deuterium concentration of approximately 148 parts per million. 

Dual-Enzyme Processes. In some cases, especially in symp production in Europe, a Hquefaction process is used that incorporates both a thermostable enzyme and a high temperature heat treatment. This type of process provides better hydrolyzate tilterabiHty than that attained in an acid Hquefaction process (9). Consequendy, dual-enzyme processes were developed that utilized multiple additions of either B. licheniformis or B. stearothermophilus a-amylase and a heat treatment step (see Eig. 1). 

In these processes, the starch slurry is prepared in the same manner as in the low temperature process. In a dual-enzyme/dual-heating process, the steps ate the same as the low temperature process until the completion of the second-stage reaction. Then, a 2—5-min heat treatment foUowed by a second enzyme addition and another reaction step is employed. In a dual-enzyme/single-heating process, the starch slurry is immediately heated to 145—150°C for one minute or less. Although the enzyme is rapidly inactivated, sufficient hydrolysis takes place to provide a partially thinned hydrolyzate that can be pumped to a second stage where additional enzyme is added and the reaction continued at 95—100°C for 20—30 minutes. The temperature is then lowered for the remainder of the reaction. 

In the dual-temperature H2O/H2S process (61,62), exchange of deuterium between H20(l) and H2S(g) is carried out at pressures of ca 2 MPa (20 atm). At elevated temperatures deuterium tends to displace hydrogen in the hydrogen sulfide and thus concentrates in the gas. At lower temperatures the driving force is reversed and the deuterium concentrates in H2S in contact with water on the tiquid phase. 

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). 

Fig. 1. Simplified flow diagrams for H2S/H2O heavy water processes, (a) Dual-temperature system where the pressure is 1.90 MPa (b) siagle-temperature...

A variant of the H2/NH2 chemical exchange process uses alkyl amines in place of ammonia. Hydrogen exchange catalyzed by NH2 is generaHy faster using alkyl amines than ammonia, and a dual-temperature flow sheet for a H2/CH2NH2 process has been developed (69). 

The potential for electrochemical corrosion in a boiler results from an inherent thermodynamic instability, with the most common corrosion processes occurring at the boiler metal surface and the metal-BW interface (Helmholtz double layer). These processes may be controlled relatively easily in smaller and simpler design boilers (such as dual-temperature, LPHW heating, and LP steam boiler systems) by the use of various anodic inhibitors. 

Dual nickel, 9 820—821 Dual-pressure processes, in nitric acid production, 17 175, 177, 179 Dual-solvent fractional extraction, 10 760 Dual Ziegler catalysts, for LLDPE production, 20 191 Dubinin-Radushkevich adsorption isotherm, 1 626, 627 Dubnium (Db), l 492t Ductile (nodular) iron, 14 522 Ductile brittle transition temperature (DBTT), 13 487 Ductile cast iron, 22 518—519 Ductile fracture, as failure mechanism, 26 983 Ductile iron... 

Dual Temperature Exchange The GS Process for Deuterium Enrichment... 

In the single-pressure and dual-pressure processes, the catalyst volatilizes at a rate determined by the converter exit-gas temperature. Experimental work indicates that the rate loss of catalyst (without a catalyst recovery system) is approximately three times more rapid at 973°C than at 866°C (Ref. PT18). From plant operation data, the loss from a dual-pressure converter (operating at 866°C) is estimated at about 0.10 g/tonne of 100% acid, and from a single-pressure converter (operating at 937°C) it is estimated at about 0.38 g/tonne of 100% acid. 

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