On-line Gas Analysis in Urea Plants0 pages

Chemical

Application Note

On-line Gas Analysis in Urea Plants
Overview
In 2012, urea plants produced nearly 184 million tons worldwide.
Over 90 % of it is used as a nitrogen release fertilizer. More than
90 % of world industrial production of urea is destined for use as a
nitrogen release fertilizer. Urea has the highest nitrogen content
of all solid nitrogenous fertilizers, therefore it has the lowest
transportation costs per unit of nitrogen nutrient. Other uses
can be found in the chemical industry with the manufacturing of
urea-formaldehyde resins and urea-melamine-formaldehyde, ior
in explosives for making urea nitrate among other applications.
Additionally, urea can also be used for SNCR and SCR reactions
to reduce the NOx pollutants in exhaust gases made from Diesel
combustion, dual fuel, and lean-burn natural gas engines.

X2FD flame proof housing for installation in hazardous area.

Background
Urea is produced at an industrial scale from the reaction of synthetic
ammonia (NH3) with carbon dioxide (CO2). As large quantities of
carbon dioxide are produced during the ammonia manufacturing
process as a byproduct from steam reforming of hydrocarbons, urea
production plants are almost always located adjacent to the site
where the ammonia is manufactured. For example:
1.tCarbamate formation: fast exothermic reaction of liquid NH3
with gaseous CO2 at high temperature and pressure to form
ammonium carbamate (H2N–COONH4)
2.tUrea conversion: slower endothermic decomposition of
ammonium carbamate into urea and water
The overall conversion of NH3 and CO2 to urea is exothermic.
Ammonium carbamate solutions are notoriously corrosive towards
metallic materials of construction, even the more resistant forms of
stainless steel. Corrosion can be minimized by continuously injecting
a minor amount of oxygen (as air) into the plant to establish and
maintain a passive oxide layer on exposed stainless steel surfaces.

Applications
Carbon dioxide feed is recovered from ammonia synthesis gas and
contains traces of hydrogen which can mingle with the passivation
air to form an explosive mixture if allowed to accumulate in
the plant. Therefore the H2 content of the CO2 feed need to be
monitored.

In total recycle NH3 stripping urea process, ammonia return
gas from the urea reactor needs to be scrubbed of CO2 before
entering the ammonia compressor to avoid precipitant formation.
Compressor fouling contributes heavily to significant and unplanned
maintenance costs. Efficiency checks of the CO2 scrubbers greatly
minimize the ammonium carbamate and ammonium carbonate
fouling potential.

Solutions
The H2 concentration in CO2 feedstock can go up to several thousand
ppm with a normal value of 200 ppm. Normal values for N2 are
2.9 %, 0.88 % O2, 400 ppm Ar, 50 ppm CH4 and 2000 ppm H2O. All
concentrations do not show much variation. Therefore a 0–1 % H2
TCD measurement which is calibrated in a CO2 background can
be applied. The detection limit is < 100 ppm. For installation in
hazardous area the TCD is packaged in a flame proofed housing of
the X-STREAM series. For lower H2 concentration with a lowest range
of 0–1000 ppm H2 measurements a special TCD solution in the MLT2
field housing is available.
A flowing reference with CO2 is used to minimize drift effects.
Sample gas flow and reference gas flow are controlled either with a
pressure regulator and capillary or mass flow controllers. A purged
system for installation in hazardous area can be configured. Both
TCD solutions have to be calibrated in CO2 background.