Revamp of Ammonia Plant

Background
The customer is a leading manufacturer of ammonia, urea, di-methyl-formamide and ammonium bi-carbonate, among others.

Process Overview
The ammonia and urea production units generate their hydrogen (H₂), di-nitrogen (N₂) and carbon dioxide (CO₂) feeds through a common reformer.

Outlet gases from the reformer are passed through the absorber, where hydrogen along with other light gases are separated, while the carbon dioxide is removed through absorption in the solution. Hydrogen and other light gases along with slipped carbon dioxide from the top of the absorber are passed to the methanation section. The methanator converts the residual carbon dioxide to methane to increase the catalyst life of the ammonia reactor.

The rich solution from the absorber is then stripped in the regenerator and the carbon dioxide gas is used to produce urea. The lean solution from the regenerator bottom is sent back to the absorber.

Customer Requirements
In the existing condition, the carbon dioxide slippage from the absorber was very high (~1200 ppm). This led to an increase in the load on methanator and also simultaneously increasing the volume of purge stream from ammonia loop.

The customer wanted to revamp the system in order to:

• Reduce slippage of CO₂
  
• Increase plant capacity without sacrificing absorber efficiency

Problem analysis
During the study of the existing system it was noted that Pall Rings were used as mass transfer media in the packed towers. The Pall Rings inherently have a drawback of higher pressure drop and performance limitations for mass transfer. This was resulting in increased load on the compressor, finally leading to slippage of excess carbon dioxide.

Solutions Provided
Hydraulic rating of existing towers along with the tower internals was done using Medal-Pak^® and following results were noted:

• The pressure drop was found to be appreciably lower than that of Pall Rings
• The capacity could be increased by 20%
• Minor modification of the existing internals to handle higher loads was also suggested

Further, based on the process overview it was predicted that the slippage of carbon dioxide would be in the range from ~ 1200 ppm.

Accordingly, Medal-Pak^® tower packings were manufactured, supplied and installed. During the shutdown the internals were modified on site.

Results achieved

• After the packing replacement, CO₂ slip decreased to 500 ppm at higher capacity of 105%
• Steam consumption in re-generator decreased to 10%