Alternative Control Techniques Document— Nitric And Adipic Acid

. The process control device consists of a three-stage
absorption column with gas and liquid chillers on the feed gas
and recirculated solvents. Liquid ammonia or some other form of
refrigeration is used as the cooling medium. The chemical
reaction mechanisms proposed for urea scrubbing are as follows:
HN0 + CO(NH ) X N + HNCO + H O Eq. 8
2 2 2 2 2
HNCO + HNO X N + CO + H O Eq. 9
2 2 2 2
HNCO + H O + H + X NH + CO Eq. 10
2 4 2
Under actual process operating conditions, the last reaction
listed above predominates so that the overall reaction is:
HNO + CO (NH ) + HNO 6 N + CO + NH NO + H O Eq. 11
2 2 2 3 2 2 4 3 2
In the MASAR process, absorber tail gas is first cooled in a
gas chiller, where condensation occurs and forms nitric acid.
Normal plant absorber feedwater is chilled in the top section of
the MASAR absorber and is then fed to the bottom section, where
it flows countercurrent to the incoming chilled tail gas in the
packed bed. After additional NO is scrubbed from the tail gas,
x
the scrubbing water is recirculated through a chiller to remove
reaction heat; this weak acid stream is used as feed to the
nitric acid plant absorber. In the middle section of the MASAR
absorber, the tail gas is scrubbed with the urea-containing
solution, forming nitric acid and nitrous acid that reacts to
form CO(NH ), N , and H O. Recirculation of the scrubbing
2 2 2
solution causes the concentration of nitric acid and ammonium
nitrate to rise. Therefore, a bleed stream is required to keep
the system in balance. Makeup urea/water solution is fed to the
scrubbing system at a rate sufficient to maintain a specified
minimum urea residual content. To maintain temperature control
in the middle section, the recirculated scrubbing solution is
pumped through a chiller to remove the heat of reaction. Prior
to leaving the MASAR unit, the tail gas is again scrubbed with
plant absorber feed water in the top section. 5
This process has been reported to reduce NO emissions from
x
4,000 to 100 ppm (26 to 0.7 kg per metric ton [57 to 1.4 lb/ton])
8
and can theoretically be designed for zero liquid discharge. In
practice, however, liquid blowdown of 16 kg/hr (35 lb/hr) of urea
5-37