Vapor-phase ammonia-injection

In the vapor-phase ammonia-injection process, water (or steam) and gaseous ammonia are injected into the stack gas while the gas is at some temperature (>160°F) above the decomposition temperatiure of ammonium sulfite, the principal product (140° to 158°F) (2). After the water is vaporized and the reactants are thoroughly mixed, the gas is cooled below 140°F and the finely divided salt particles separate from the gas as a smoke or fume dp = 0.01-1.0/un). The entrained solids, salt particles, and fly ash are then recovered concurrently. 

A vapor-phase ammmonia-injection process for SO2 removal is being developed at the Morgantown Energy Research Center. In laboratory research with simulated stack gas containing 4200 ppm SO2 (equivalent to 6.0% sulfur in coal), essentially complete removal of SO2 from the gas phase was effected by ammonia injection (i). Preliminary work has since proceeded with a small pilot-scale installation in which the sulfur products from the vapor-phase reaction are removed in a water scrubber. This paper presents additional data from the laboratory work and the pilot-scale installation. 

The water was not injected through a spray nozzle into the pipe. Hence, the washwater was not dispersed. Only the periphery of the water was contacting the vapor phase. The major portion of the washwater remained basic due to the excess of ammonia dissolved in the water. 

Only the peripheral area of the injected water came into intimate contact with the acidic vapor phase in the pipe. Hence, only this peripheral water became acidic as the ammonia in the water was neutralized by the H2S, CO2, and cyanides in the vapor. 

Although small quantities of sour water may be disposed of by injection into deep wells or by adding to the plant waste water disposal system, these are not viable options for most plants. Normally the sour water is either processed in a sour water stripper (SWS), which produces a vapor phase containing both ammonia and acid gases, or it is selectively stripped in a two-colunon system such as the Chevron WWT process, which produces separate ammonia and hydrogen sulflde-rich gas streams. 

Ammonia is the most common material because of its high neutralizing power, low unit cost, easy availability, and convenience of handling. It may be injected as a liquid under cylinder pressure and flashed into the vapor phase of the crude still. Upon condensation of the vapors, ammonia will dissolve into the condensate water to effect an increase in its pH. As additional water condenses down-stream of the initial point, it will be in equilibrium with ammonia gas in the condensing hydrocarbon and water vapors. 

The materials of constmction of the radiant coil are highly heat-resistant steel alloys, such as Sicromal containing 25% Cr, 20% Ni, and 2% Si. Triethyi phosphate [78-40-0] catalyst is injected into the acetic acid vapor. Ammonia [7664-41-7] is added to the gas mixture leaving the furnace to neutralize the catalyst and thus prevent ketene and water from recombining. The cmde ketene obtained from this process contains water, acetic acid, acetic anhydride, and 7 vol % other gases (mainly carbon monoxide [630-08-0][124-38-9] ethylene /74-< 3 -/7, and methane /74-< 2-<7/). The gas mixture is chilled to less than 100°C to remove water, unconverted acetic acid, and the acetic anhydride formed as a Hquid phase (52,53). 

If we now inject additional ammonia into the container, a new set of equilibrium concentrations will eventually be established, with higher concentrations in each phase than were initially obtained. In this manner, we can eventually obtain the complete relationship between the equilibrium concentrations in both phases. If the ammonia is designated as substance A, the equilibrium mole fractions in the gas (yA) and liquid ( A) give rise to an equilibrium-distribution curve as shown in Figure 3.1. This curve results irrespective of the amounts of air and water that we start with, and is influenced only by the temperature and pressure of the system. It is important to note that at equilibrium the concentrations in the two phases are not equal instead, the chemical potential of the ammonia is the same in both phases. It is this equality of chemical potentials which causes the net transfer of ammonia to stop. The curve of Figure 3.1 does not, of course, show all the equilibrium concentrations existing within the system. For example, water will partially vaporize into the gas phase, the components of the air will also dissolve to a small extent into the liquid, and equilibrium concentrations for these substances will also be established. For the moment, we need not consider these equilibria, since they are of minor importance. 

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