Absorber absorption reactor

Cases have been observed where the isotopic line absorption profiles completely overlap, e.g. boron-10 and -11 in a krypton-filled lamp at 249.7 nm [244]. Hannaford and Lowe [245] later showed that this was caused by an unusually large Doppler half-width induced by the fill-gas, and, if neon is used, the 208.9 and 209.0 nm lines can allow the determination of boron-10 and boron-11 isotope ratios. The 208.89/208.96 nm doublet was found to be more useful than the 249.68/249.77 nm doublet. Enriched isotope hollow-cathode lamps were used as sources. A sputtering cell was preferred to a nitrous oxide/acetylene flame as the atom reservoir, as it could be water-cooled to reduce broadening and solid samples could be used, thus avoiding the slow dissolution in nitric acid of samples of boron-10 used as a neutron absorber in reactor technology. 

Two books deal almost exclusively with the subject of mass transfer with chemical reaction, the admirably clear expositions of Astarita (A6) and Danckwerts (D2). Since then a flood of theoretical and experimental work has been reported on gas absorption and related separations. The principal object of this chapter is to present techniques, results, and opinions published mainly during the last 6 or 7 years on mass-transfer coefficients and interfacial areas in most types of absorbers and reactors. This necessitates some review of mass transfer with and without chemical reaction in the first section, and comments about the simulation of industrial reactors by laboratory-scale apparatus in the concluding section. Although many gas-liquid reactions are accompanied by a rise in temperature that may be great enough to affect the rate of gas absorption, our attention here is confined to cases where the rise in temperature does not affect the absorption rate. This latter topic (treated by references B20, TIO, S3, T3, V5) could justify another complete chapter. 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

The process options reflect the broad range of compositions and gas volumes that must be processed. Both batch processes and continuous processes are used. Batch processes are used when the daily production of sulfur is small and of the order of 10 kg. When the daily sulfur production is higher, of the order of 45 kg, continuous processes are usually more economical. Using batch processes, regeneration of the absorbant or adsorbant is carried out in the primary reactor. Using continuous processes, absorption of the acid gases occurs in one vessel and acid gas recovery and solvent regeneration occur in a separate reactor. 

The nuclear chain reaction can be modeled mathematically by considering the probable fates of a typical fast neutron released in the system. This neutron may make one or more coUisions, which result in scattering or absorption, either in fuel or nonfuel materials. If the neutron is absorbed in fuel and fission occurs, new neutrons are produced. A neutron may also escape from the core in free flight, a process called leakage. The state of the reactor can be defined by the multiplication factor, k, the net number of neutrons produced in one cycle. If k is exactly 1, the reactor is said to be critical if / < 1, it is subcritical if / > 1, it is supercritical. The neutron population and the reactor power depend on the difference between k and 1, ie, bk = k — K closely related quantity is the reactivity, p = bk jk. i the reactivity is negative, the number of neutrons declines with time if p = 0, the number remains constant if p is positive, there is a growth in population. 

Hydrides. Zirconium hydride [7704-99-6] in powder form was produced by the reduction of zirconium oxide with calcium hydride in a bomb reactor. However, the workup was hazardous and many fires and explosions occurred when the calcium oxide was dissolved with hydrochloric acid to recover the hydride powder. With the ready availabiHty of zirconium metal via the KroU process, zirconium hydride can be obtained by exothermic absorption of hydrogen by pure zirconium, usually highly porous sponge. The heat of formation is 167.4 J / mol (40 kcal/mol) hydrogen absorbed. 

There are many processes used in tail-gas treating. The Sulfreen and the Cold Bed Absorption (CBA) processes use two psirallel reactors in a cycle, where one reactor operates below the sulfur dew point to absorb the sulfur while the second is regenerated with heat to recover molten sulfur, tiven though sulfur recoveries with the additional reactors are normally 99-99.5% of the inlet stream to the Claus unit, incineration of the outlet gas may still be required. 

In a continuous steady state reactor, a slightly soluble gas is absorbed into a liquid in which it dissolves and reacts, the reaction being second order with respect to the dissolved gas. Calculate the reaction rate constant on the assumption that the liquid is semi-infinite in extent and that mass transfer resistance in the gas phase is negligible. The diffusivity of the gas in the liquid is 10" 8 m2/s, the gas concentration in the liquid falls to one half of its value in the liquid over a distance of 1 mm, and the rate of absorption at the interface is 4 x 10"6 kmol/m2 s. 

Example 11.7 Carbon dioxide is sometimes removed from natural gas by reactive absorption in a tray column. The absorbent, typically an amine, is fed to the top of the column and gas is fed at the bottom. Liquid and gas flow patterns are similar to those in a distillation column with gas rising, liquid falling, and gas-liquid contacting occurring on the trays. Develop a model for a multitray CO2 scrubber assuming that individual trays behave as two-phase, stirred tank reactors. 

Piston Flow in Contact with a CSTR. A liquid-phase reaction in a spray tower is conceptually similar to the transpired-wall reactors in Section 3.3. The liquid drops are in piston flow but absorb components from a well-mixed gas phase. The rate of absorption is a function of as it can be in a transpired-wall reactor. The component balance for the piston flow phase is... 

Fig 18. Experimental trickle-bed system A, tube bundle for liquid flow distribution B, flow distribution packing of glass helices C, activated carbon trickle bed 1, mass flow controllers 2, gas or liquid rotameters, 3, reactor (indicating point of gas phase introduction) 4, overflow tank for the liquid phase feed 5, liquid phase hold-up tank 6, absorber pump 7, packed absorption column for saturation of the liquid phase 8, gas-liquid disengager in the liquid phase saturation circuit. (Figure from Haure et ai, 1989, with permission, 1989 American Institute of Chemical Engineers.)... 

The reactor products are fed to a condenser where the chlorobenzenes and unreacted benzene are condensed. The condensate is separated from the noncondensable gases in a separator. The non-condensables, hydrogen chloride and unreacted chlorine, pass to an absorption column where the hydrogen chloride is absorbed in water. The chlorine leaving the absorber is recycled to the reactor. The liquid phase from the separator, chlorobenzenes and unreacted benzene, is fed to a distillation column, where the chlorobenzenes are separated from the unreacted benzene. The benzene is recycle to the reactor. 

Zirconium alloys are used as in-reactor materials because of their very low neutron absorption, high strength and excellent corrosion resistance. However, they do absorb hydrogen freely. 

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