High efficiency combined process

Best option for revamping Total Recycle plants with capacity increase up to 60%

Key features

• CO2 conversion up to 72%
• Steam consumption 900 kg/MT urea

This process is the best revamping option for Total Recycle plants when the target is increasing the capacity by 50-60% over the original nameplate value.

Significantly, the High Efficiency Combined (HEC) process operates the existing HP synthesis reactor as a once-through synthesis step fed only with ammonia and carbon dioxide. Since no water is recycled, a very high CO2 conversion is achieved. An HP condenser is added upstream of the once-through reactor to recover part of the heat of carbamate formation by producing LP steam.

The small amount of residual carbamate is decomposed, condensed and recycled as aqueous solution to a new synthesis section in parallel to the once-through reactor. This section comprises a "secondary" urea reactor and an HP carbamate decomposer. The unreacted effluent from the "primary" once-through reactor is decomposed in the HP decomposer with 20 bar steam and the generated vapours are routed back to the "secondary" reactor, whereas the urea solution from the bottom is sent to the existing downstream MP and LP decomposition stages. The "secondary" reactor is fed only with vapours from the HP decomposer and the recycled carbamate. The outlet urea and carbamate solution join the urea solution from the stripper bottom.

The key advantage of the HEC process, stemming from its high conversion efficiency, is that the load on the back-end equipment is not significantly increased, even when production increase is as high as 60% of the original capacity. Therefore, the major modifications to the original lay-out are limited to the high-pressure section whilst the back end is much less affected.

The efficiency of the process, in terms of steam consumption, is also significantly improved in comparison with conventional total recycle processes. The typical specific consumption for a HEC process is in the range of 900 kg/MT urea.

Figure: Block flow diagram of total recycle urea plant revamped with HEC technology