July 9, 2014

On-Line Gas Analysis in Urea Plants

By Michael Kamphus

Today, we’re going to take a look at gas analysis in urea plants. Urea is produced on an industrial scale from 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.

The basic process, developed in 1922, is called the Bosch Meiser urea process, which consists of two steps:

  1. Carbamate formation: fast exothermic reaction of liquid NH3 with gaseous CO2 at high temperature and pressure to form ammonium carbamate (H2N-COONH4)
  2. Urea 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.

Typical Challenges
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 needs to be monitored.

In the 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.

Recommended 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 areas, the TCD is packaged in a flame proofed housing of the X-STREAM series. For lower H2 concentration measurements, with a lowest range of 0-1000 ppm H2, a special TCD solution in the MLT2 field housing is available.

What other urea plant challenges have you experienced?