Partial pressure ammonia

Because the ammonia synthesis reaction is an equiUbrium, the quantity of ammonia depends on temperature, pressure, and the H2 to-N2 ratio. At 500°C and 20.3 MPa (200 atm), the equiUbrium mixture contains 17.6% ammonia. The ammonia formed is removed from the exit gases by condensation at about —20° C, and the gases are recirculated with fresh synthesis gas into the reactor. The ammonia must be removed continually as its presence decreases both the equiUbrium yield and the reaction rate by reducing the partial pressure of the N2—H2 mixture. 

Ammonium Phosphates. In the manufacture of ammonium phosphates, an atmosphere of ammonia may need to be maintained because the partial pressure of ammonia rises rapidly as either the temperature or the NH2/P20 mole ratio of the reaction mass increases. Phosphoric acid reacts quickly with ammonia vapor and is used in multistage reactor systems as a scmbber fluid to prevent NH emissions and recover ammonia values. For example, H PO scmbbing of coke-oven off-gases produces ammonium phosphates of relatively good purity. 

Conditions cited for Rh on alumina hydrogenation of MDA are much less severe, 117 °C and 760 kPA (110 psi) (26). With 550 kPa (80 psi) ammonia partial pressure present ia the hydrogenation of twice-distilled MDA employing 2-propanol solvent at 121°C and 1.3 MPa (190 psi) total pressure, the supported Rh catalyst could be extensively reused (27). Medium pressure (3.9 MPa = 566 psi) and temperature (80°C) hydrogenation usiag iridium yields low trans trans isomer MDCHA (28). Improved selectivity to aUcychc diamine from MDA has been claimed (29) for alumina-supported iridium and rhodium by iatroduciag the tertiary amines l,4-diazabicyclo[2.2.2]octane [280-57-9] and quiaucHdine [100-76-5]. 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 sHp required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availabiUty of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

In almost all modem plants, the ammonia is recovered by condensation and at modern synthesis pressures, ammonia is usually the source of refrigeration required. In order to maintain a high partial pressure of reactants, inerts entering with the make-up gas are normally removed using a purge stream. 

Pn, = partial pressure of nitrogen Pnh3 = partial pressure of ammonia (-1 ) = rate of reaetion of nitrogen, moles of nitrogen disappearing per unit time per unit mass of eatalyst... 

The equilibrium eonstant for ammonia synthesis is expressed as a funetion of the partial pressure as... 

Figure 9-76. Partial pressure of ammonia over aqueous solutions of ammonia. Used by permission of Leva, M., Tower Packings and Packed Tower Design, 2nd Ed., U.S. Stoneware Co. (now, Norton Chemical Process Products Corp.) (1953).

In a sealed flask at 25°C are 10.0 g ofNH4HS, ammonia with a partial pressure of0.692 atm, and H2S with a partial pressure of0.0532 atm. When equilibrium is established, it is found that the partial pressure of ammonia has increased by 12.4%. Calculate K for the decomposition of NH4HS at 25°C. 

Calculate the equilibrium partial pressure of hydrogen if the equilibrium partial pressures of ammonia and nitrogen are 0.015 atm and 1.2 atm, respectively. 

The equilibrium constant for the decomposition at a certain temperature is 25. Calculate the partial pressures of all the gases at equilibrium if ammonia with a pressure of 1.00 atm is sealed in a 3.0-L flask. 

Ammonia is absorbed in water from a mixture with air using a column operating at I bar and 295 K. The resistance to transfer may be regarded as lying entirely within the gas phase. At a point in the column, the partial pressure of the ammonia is 7.0 kN/m2. The back pressure at the water interface is negligible and the resistance to transfer may be regarded as lying in a stationary gas film 1 mm thick. If the diffusivity of ammonia in air is 2.36 x 10 5 m2/s, what is the transfer rate per unit area at that point in the column How would the rate of transfer be affected if the ammonia air mixture were compressed to double the pressure ... 

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

Neglect the partial pressure attributable to ammonia in the bulk solution. 

In this example, only one of the reagents has a concentration that can vaiy, and each stoichiometric coefficient is one. What happens for a more complicated reaction Consider the synthesis of ammonia carried out in a pressurized reactor containing N2, H2, and NH3 at partial pressures different from 1 bar ... 

We will list the elementary steps and decide which is rate-limiting and which are in quasi-equilibrium. For ammonia synthesis a consensus exists that the dissociation of N2 is the rate-limiting step, and we shall make this assumption here. With quasi-equilibrium steps the differential equation, together with equilibrium condition, leads to an expression for the coverage of species involved in terms of the partial pressures of reactants, equilibrium constants and the coverage of other intermediates. 

Figure 7.21 shows the results for the ammonia synthesis on real catalysts in a reactor. The surface is predominantly covered by atomic nitrogen and by NH intermediates. This actually limits the rate of the reaction as soon as an appreciable partial pressure of ammonia has built up. In fact, ammonia poisons the reaction. 

Fig. 4.2 Comparison of amino acid yields using CH4, CO and CO2 as carbon sources with the addition of varying amounts of H2. The yields were calculated on the basis of the amount of carbon present in the reaction mixture. In all cases, the partial pressures of nitrogen, methane, carbon monoxide and carbon dioxide were lOOmmHg. For the reactions using nitrogen, the reaction vessel contained 100 mL of water, but no ammonia. Reactions involving nitrogen and ammonia were carried out using 100 mL of ammonium chloride (0.05 M). The electrical discharge experiments took 48 hours at room temperature (Schlesinger and Miller, 1983)...

Data were taken of the decomposition of ammonia on a tungsten wire at 856 C with two different starting pressures (Hinshelwood Burk, J Chem Soc 127 1105, 1925). Time is tabulated against the change in partial pressure,... 

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